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)
#endif
#endif
    strUsage += HelpMessageOpt("-whitebind=<addr>", _("Bind to given address and whitelist peers connecting to it. Use [host]:port notation for IPv6"));
    strUsage += HelpMessageOpt("-whitelist=<IP address or network>", _("Whitelist peers connecting from the given IP address (e.g. 1.2.3.4) or CIDR notated network (e.g. 1.2.3.0/24). Can be specified multiple times.") +
        " " + _("Whitelisted peers cannot be DoS banned and their transactions are always relayed, even if they are already in the mempool, useful e.g. for a gateway"));
#ifdef ENABLE_WALLET
    strUsage += GetWalletHelpString(showDebug);
#endif
#if ENABLE_ZMQ
    strUsage += HelpMessageGroup(_("ZeroMQ notification options:"));
    strUsage += HelpMessageOpt("-zmqpubhashblock=<address>", _("Enable publish hash block in <address>"));
    strUsage += HelpMessageOpt("-zmqpubhashtx=<address>", _("Enable publish hash transaction in <address>"));
    strUsage += HelpMessageOpt("-zmqpubrawblock=<address>", _("Enable publish raw block in <address>"));
    strUsage += HelpMessageOpt("-zmqpubrawtx=<address>", _("Enable publish raw transaction in <address>"));
#endif
    strUsage += HelpMessageGroup(_("Debugging/Testing options:"));
    if (showDebug)
    {
        strUsage += HelpMessageOpt("-checkblocks=<n>", strprintf(_("How many blocks to check at startup (default: %u, 0 = all)"), DEFAULT_CHECKBLOCKS));
        strUsage += HelpMessageOpt("-checklevel=<n>", strprintf(_("How thorough the block verification of -checkblocks is (0-4, default: %u)"), DEFAULT_CHECKLEVEL));
        strUsage += HelpMessageOpt("-checkblockindex", strprintf("Do a full consistency check for mapBlockIndex, setBlockIndexCandidates, chainActive and mapBlocksUnlinked occasionally. Also sets -checkmempool (default: %u)", defaultChainParams->DefaultConsistencyChecks()));
        strUsage += HelpMessageOpt("-checkmempool=<n>", strprintf("Run checks every <n> transactions (default: %u)", defaultChainParams->DefaultConsistencyChecks()));
        strUsage += HelpMessageOpt("-checkpoints", strprintf("Disable expensive verification for known chain history (default: %u)", DEFAULT_CHECKPOINTS_ENABLED));
        strUsage += HelpMessageOpt("-disablesafemode", strprintf("Disable safemode, override a real safe mode event (default: %u)", DEFAULT_DISABLE_SAFEMODE));
        strUsage += HelpMessageOpt("-deprecatedrpc=<method>", "Allows deprecated RPC method(s) to be used");
        strUsage += HelpMessageOpt("-testsafemode", strprintf("Force safe mode (default: %u)", DEFAULT_TESTSAFEMODE));
        strUsage += HelpMessageOpt("-dropmessagestest=<n>", "Randomly drop 1 of every <n> network messages");
        strUsage += HelpMessageOpt("-fuzzmessagestest=<n>", "Randomly fuzz 1 of every <n> network messages");
        strUsage += HelpMessageOpt("-stopafterblockimport", strprintf("Stop running after importing blocks from disk (default: %u)", DEFAULT_STOPAFTERBLOCKIMPORT));
        strUsage += HelpMessageOpt("-stopatheight", strprintf("Stop running after reaching the given height in the main chain (default: %u)", DEFAULT_STOPATHEIGHT));