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("-dbcache=<n>", strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache));
    strUsage += HelpMessageOpt("-debuglogfile=<file>", strprintf(_("Specify location of debug log file. Relative paths will be prefixed by a net-specific datadir location. (default: %s)"), DEFAULT_DEBUGLOGFILE));
    if (showDebug)
        strUsage += HelpMessageOpt("-feefilter", strprintf("Tell other nodes to filter invs to us by our mempool min fee (default: %u)", DEFAULT_FEEFILTER));
    strUsage += HelpMessageOpt("-loadblock=<file>", _("Imports blocks from external blk000??.dat file on startup"));
    strUsage += HelpMessageOpt("-maxmempool=<n>", strprintf(_("Keep the transaction memory pool below <n> megabytes (default: %u)"), DEFAULT_MAX_MEMPOOL_SIZE));
    strUsage += HelpMessageOpt("-maxorphantx=<n>", strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS));
    strUsage += HelpMessageOpt("-mempoolexpiry=<n>", strprintf(_("Do not keep transactions in the mempool longer than <n> hours (default: %u)"), DEFAULT_MEMPOOL_EXPIRY));
    if (showDebug) {
        strUsage += HelpMessageOpt("-minimumchainwork=<hex>", strprintf("Minimum work assumed to exist on a valid chain in hex (default: %s, testnet: %s)", defaultChainParams->GetConsensus().nMinimumChainWork.GetHex(), testnetChainParams->GetConsensus().nMinimumChainWork.GetHex()));
    }
    strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
        -GetNumCores(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
    strUsage += HelpMessageOpt("-persistmempool", strprintf(_("Whether to save the mempool on shutdown and load on restart (default: %u)"), DEFAULT_PERSIST_MEMPOOL));
#ifndef WIN32
    strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file. Relative paths will be prefixed by a net-specific datadir location. (default: %s)"), BITCOIN_PID_FILENAME));
#endif
    strUsage += HelpMessageOpt("-prune=<n>", strprintf(_("Reduce storage requirements by enabling pruning (deleting) of old blocks. This allows the pruneblockchain RPC to be called to delete specific blocks, and enables automatic pruning of old blocks if a target size in MiB is provided. This mode is incompatible with -txindex and -rescan. "
            "Warning: Reverting this setting requires re-downloading the entire blockchain. "
            "(default: 0 = disable pruning blocks, 1 = allow manual pruning via RPC, >%u = automatically prune block files to stay under the specified target size in MiB)"), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
    strUsage += HelpMessageOpt("-reindex", _("Rebuild chain state and block index from the blk*.dat files on disk"));
    strUsage += HelpMessageOpt("-reindex-chainstate", _("Rebuild chain state from the currently indexed blocks"));
#ifndef WIN32
    strUsage += HelpMessageOpt("-sysperms", _("Create new files with system default permissions, instead of umask 077 (only effective with disabled wallet functionality)"));
#endif
    strUsage += HelpMessageOpt("-txindex", strprintf(_("Maintain a full transaction index, used by the getrawtransaction rpc call (default: %u)"), DEFAULT_TXINDEX));
    strUsage += HelpMessageGroup(_("Connection options:"));
    strUsage += HelpMessageOpt("-addnode=<ip>", _("Add a node to connect to and attempt to keep the connection open (see the `addnode` RPC command help for more info)"));
    strUsage += HelpMessageOpt("-banscore=<n>", strprintf(_("Threshold for disconnecting misbehaving peers (default: %u)"), DEFAULT_BANSCORE_THRESHOLD));
    strUsage += HelpMessageOpt("-bantime=<n>", strprintf(_("Number of seconds to keep misbehaving peers from reconnecting (default: %u)"), DEFAULT_MISBEHAVING_BANTIME));