QMP: Drop old client argument checker

Previous two commits added qmp_check_client_args(), which
fully replaces this code and is way better.

It's important to note that the new checker doesn't support
the '/' arg type. As we don't have any of those handlers
converted to QMP, this is just dead code.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
