monitor: Drop do_qmp_capabilities()'s superfluous QMP check

Superfluous since commit 30f5041 removed it from HMP.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
