monitor: simplify monitor_qmp_setup_handlers_bh

When we reach monitor_qmp_setup_handlers_bh() we must be using the
IOThread then, so no need to check against it any more.  Instead, we
assert.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180815133747.25032-2-peterx@redhat.com>
[Insufficiently useful comment dropped]
Signed-off-by: Markus Armbruster <armbru@redhat.com>