monitor: drop unused monitor_cur_is_qmp

The previous patch dropped the only remaining use of
monitor_cur_is_qmp.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20260706135824.2623960-19-berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

stubs/monitor-core.c

@@ -7,11 +7,6 @@ Monitor *monitor_cur(void)
     return NULL;
 }
 
-bool monitor_cur_is_qmp(void)
-{
-    return false;
-}
-
 Monitor *monitor_set_cur(Coroutine *co, Monitor *mon)
 {
     return NULL;