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("-limitancestorcount=<n>", strprintf("Do not accept transactions if number of in-mempool ancestors is <n> or more (default: %u)", DEFAULT_ANCESTOR_LIMIT));
        strUsage += HelpMessageOpt("-limitancestorsize=<n>", strprintf("Do not accept transactions whose size with all in-mempool ancestors exceeds <n> kilobytes (default: %u)", DEFAULT_ANCESTOR_SIZE_LIMIT));
        strUsage += HelpMessageOpt("-limitdescendantcount=<n>", strprintf("Do not accept transactions if any ancestor would have <n> or more in-mempool descendants (default: %u)", DEFAULT_DESCENDANT_LIMIT));
        strUsage += HelpMessageOpt("-limitdescendantsize=<n>", strprintf("Do not accept transactions if any ancestor would have more than <n> kilobytes of in-mempool descendants (default: %u).", DEFAULT_DESCENDANT_SIZE_LIMIT));
        strUsage += HelpMessageOpt("-vbparams=deployment:start:end", "Use given start/end times for specified version bits deployment (regtest-only)");
        strUsage += HelpMessageOpt("-addrmantest", "Allows to test address relay on localhost");
    }
    strUsage += HelpMessageOpt("-debug=<category>", strprintf(_("Output debugging information (default: %u, supplying <category> is optional)"), 0) + ". " +
        _("If <category> is not supplied or if <category> = 1, output all debugging information.") + " " + _("<category> can be:") + " " + ListLogCategories() + ".");
    strUsage += HelpMessageOpt("-debugexclude=<category>", strprintf(_("Exclude debugging information for a category. Can be used in conjunction with -debug=1 to output debug logs for all categories except one or more specified categories.")));
    strUsage += HelpMessageOpt("-help-debug", _("Show all debugging options (usage: --help -help-debug)"));
    strUsage += HelpMessageOpt("-logips", strprintf(_("Include IP addresses in debug output (default: %u)"), DEFAULT_LOGIPS));
    strUsage += HelpMessageOpt("-logtimestamps", strprintf(_("Prepend debug output with timestamp (default: %u)"), DEFAULT_LOGTIMESTAMPS));
    if (showDebug)
    {
        strUsage += HelpMessageOpt("-logtimemicros", strprintf("Add microsecond precision to debug timestamps (default: %u)", DEFAULT_LOGTIMEMICROS));
        strUsage += HelpMessageOpt("-mocktime=<n>", "Replace actual time with <n> seconds since epoch (default: 0)");
        strUsage += HelpMessageOpt("-maxsigcachesize=<n>", strprintf("Limit sum of signature cache and script execution cache sizes to <n> MiB (default: %u)", DEFAULT_MAX_SIG_CACHE_SIZE));
        strUsage += HelpMessageOpt("-maxtipage=<n>", strprintf("Maximum tip age in seconds to consider node in initial block download (default: %u)", DEFAULT_MAX_TIP_AGE));
    }
    strUsage += HelpMessageOpt("-maxtxfee=<amt>", strprintf(_("Maximum total fees (in %s) to use in a single wallet transaction or raw transaction; setting this too low may abort large transactions (default: %s)"),
        CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MAXFEE)));
    if (showDebug)
    {
        strUsage += HelpMessageOpt("-printpriority", strprintf("Log transaction fee per kB when mining blocks (default: %u)", DEFAULT_PRINTPRIORITY));
    }
    strUsage += HelpMessageOpt("-printtoconsole", _("Send trace/debug info to console instead of debug.log file"));
    strUsage += HelpMessageOpt("-shrinkdebugfile", _("Shrink debug.log file on client startup (default: 1 when no -debug)"));
    strUsage += HelpMessageOpt("-uacomment=<cmt>", _("Append comment to the user agent string"));
    AppendParamsHelpMessages(strUsage, showDebug);