init: completely remove -zapwallettxes (remaining hidden option)

The `-zapwallettxes` functionality has been removed in v0.21.0
(see commit 3340dbadd3 / PR #19671),
with the parameter being kept as hidden option, to inform users via
an exit error that `abandontransaction` should be used instead.

As any guides that still suggest to use `-zapwallettxes` would refer to
a Bitcoin Core version that is EOL since many years (i.e. <= v0.20.x),
it is highly unlikely that the error caused by the option is still
relevant for any user, hence it seems fine to remove it now.

test/lint/check-doc.py

@@ -23,7 +23,7 @@ CMD_GREP_WALLET_ARGS = r"git grep --function-context 'void WalletInit::AddWallet
 CMD_GREP_WALLET_HIDDEN_ARGS = r"git grep --function-context 'void DummyWalletInit::AddWalletOptions' -- {}".format(CMD_ROOT_DIR)
 CMD_GREP_DOCS = r"git grep --perl-regexp '{}' {}".format(REGEX_DOC, CMD_ROOT_DIR)
 # list unsupported, deprecated and duplicate args as they need no documentation
-SET_DOC_OPTIONAL = set(['-h', '-help', '-dbcrashratio', '-forcecompactdb', '-zapwallettxes'])
+SET_DOC_OPTIONAL = set(['-h', '-help', '-dbcrashratio', '-forcecompactdb'])
 
 
 def lint_missing_argument_documentation():