Since #11100, in init.cpp `-blockmaxsize` is translated to `-blockmaxweight` with the code:

```
        unsigned int max_size = gArgs.GetArg("-blockmaxsize", 0);
        if (gArgs.SoftSetArg("blockmaxweight", strprintf("%d", max_size * WITNESS_SCALE_FACTOR))) {
```
However SoftSetArg requires the argument to be specified with the leading dash (ie, as `SoftSetArg("-blockmaxweight")`) without the dash the value will not be looked up later (because the later lookups specify it with a dash).

Obvious fix is just to add the dash (`SoftSetArg("-blockmaxweight", ...)`), but if there haven't been any complaints, just removing the option entirely might be better, since current behaviour is the same as if the option is just removed.

I believe this applies to master, 0.16 and 0.15 (due to backport via #11592).

Test case to demonstrate:

```
#!/usr/bin/env python3
"""Test limiting maxblocksize"""

from test_framework.test_framework import BitcoinTestFramework
from test_framework.util import *

class MaxBlockSizeTest(BitcoinTestFramework):
    def set_test_params(self):
        self.num_nodes = 2
        self.extra_args = [["-blockmaxweight=40000"], ["-blockmaxsize=10000"]]

    def run_test(self):
        chain_height = self.nodes[0].getblockcount()
        assert_equal(chain_height, 200)
        self.nodes[0].generate(100)
        self.nodes[1].generate(100)
        self.nodes[0].generate(100) # unspendable

        blk = [None,None]
        for n in [0,1]:
            vsize = 0
            address = self.nodes[n].getnewaddress()
            txes = 0
            while vsize*4 < 84000:
                txid = self.nodes[n].sendtoaddress(address, 0.25)
                tx = self.nodes[n].getrawtransaction(txid, 1)
                vsize += tx["vsize"]
                txes += 1
            self.nodes[n].generate(1)
            blk[n] = self.nodes[n].getblock(self.nodes[n].getbestblockhash())
            self.sync_all()

        assert blk[0]["weight"] <= 40000, "blockmaxweight greater than 40000"
        assert blk[1]["size"] <= 10000 or blk[1]["weight"] <= 40000, "blockmaxsize is greater than 10000"

if __name__ == '__main__':
    MaxBlockSizeTest().main()
```

Cc @jnewbery (who picked this up via an assertion failure in #11862) @TheBlueMatt (who did the original PR)
strUsage += HelpMessageOpt("-bind=<addr>", _("Bind to given address and always listen on it. Use [host]:port notation for IPv6"));
    strUsage += HelpMessageOpt("-connect=<ip>", _("Connect only to the specified node(s); -connect=0 disables automatic connections (the rules for this peer are the same as for -addnode)"));
    strUsage += HelpMessageOpt("-discover", _("Discover own IP addresses (default: 1 when listening and no -externalip or -proxy)"));
    strUsage += HelpMessageOpt("-dns", _("Allow DNS lookups for -addnode, -seednode and -connect") + " " + strprintf(_("(default: %u)"), DEFAULT_NAME_LOOKUP));
    strUsage += HelpMessageOpt("-dnsseed", _("Query for peer addresses via DNS lookup, if low on addresses (default: 1 unless -connect used)"));
    strUsage += HelpMessageOpt("-externalip=<ip>", _("Specify your own public address"));
    strUsage += HelpMessageOpt("-forcednsseed", strprintf(_("Always query for peer addresses via DNS lookup (default: %u)"), DEFAULT_FORCEDNSSEED));
    strUsage += HelpMessageOpt("-listen", _("Accept connections from outside (default: 1 if no -proxy or -connect)"));
    strUsage += HelpMessageOpt("-listenonion", strprintf(_("Automatically create Tor hidden service (default: %d)"), DEFAULT_LISTEN_ONION));
    strUsage += HelpMessageOpt("-maxconnections=<n>", strprintf(_("Maintain at most <n> connections to peers (default: %u)"), DEFAULT_MAX_PEER_CONNECTIONS));
    strUsage += HelpMessageOpt("-maxreceivebuffer=<n>", strprintf(_("Maximum per-connection receive buffer, <n>*1000 bytes (default: %u)"), DEFAULT_MAXRECEIVEBUFFER));
    strUsage += HelpMessageOpt("-maxsendbuffer=<n>", strprintf(_("Maximum per-connection send buffer, <n>*1000 bytes (default: %u)"), DEFAULT_MAXSENDBUFFER));
    strUsage += HelpMessageOpt("-maxtimeadjustment", strprintf(_("Maximum allowed median peer time offset adjustment. Local perspective of time may be influenced by peers forward or backward by this amount. (default: %u seconds)"), DEFAULT_MAX_TIME_ADJUSTMENT));
    strUsage += HelpMessageOpt("-maxuploadtarget=<n>", strprintf(_("Tries to keep outbound traffic under the given target (in MiB per 24h), 0 = no limit (default: %d)"), DEFAULT_MAX_UPLOAD_TARGET));
    strUsage += HelpMessageOpt("-onion=<ip:port>", strprintf(_("Use separate SOCKS5 proxy to reach peers via Tor hidden services (default: %s)"), "-proxy"));
    strUsage += HelpMessageOpt("-onlynet=<net>", _("Only connect to nodes in network <net> (ipv4, ipv6 or onion)"));
    strUsage += HelpMessageOpt("-peerbloomfilters", strprintf(_("Support filtering of blocks and transaction with bloom filters (default: %u)"), DEFAULT_PEERBLOOMFILTERS));
    strUsage += HelpMessageOpt("-permitbaremultisig", strprintf(_("Relay non-P2SH multisig (default: %u)"), DEFAULT_PERMIT_BAREMULTISIG));
    strUsage += HelpMessageOpt("-port=<port>", strprintf(_("Listen for connections on <port> (default: %u or testnet: %u)"), defaultChainParams->GetDefaultPort(), testnetChainParams->GetDefaultPort()));
    strUsage += HelpMessageOpt("-proxy=<ip:port>", _("Connect through SOCKS5 proxy"));
    strUsage += HelpMessageOpt("-proxyrandomize", strprintf(_("Randomize credentials for every proxy connection. This enables Tor stream isolation (default: %u)"), DEFAULT_PROXYRANDOMIZE));
    strUsage += HelpMessageOpt("-seednode=<ip>", _("Connect to a node to retrieve peer addresses, and disconnect"));
    strUsage += HelpMessageOpt("-timeout=<n>", strprintf(_("Specify connection timeout in milliseconds (minimum: 1, default: %d)"), DEFAULT_CONNECT_TIMEOUT));
    strUsage += HelpMessageOpt("-torcontrol=<ip>:<port>", strprintf(_("Tor control port to use if onion listening enabled (default: %s)"), DEFAULT_TOR_CONTROL));
    strUsage += HelpMessageOpt("-torpassword=<pass>", _("Tor control port password (default: empty)"));
#ifdef USE_UPNP
#if USE_UPNP
    strUsage += HelpMessageOpt("-upnp", _("Use UPnP to map the listening port (default: 1 when listening and no -proxy)"));
#else
    strUsage += HelpMessageOpt("-upnp", strprintf(_("Use UPnP to map the listening port (default: %u)"), 0));