clutter: Drop CLUTTER_CLIENT_MESSAGE event

This is used nowhere and emitted nowhere. We can do without it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1623>

src/core/events.c

@@ -111,8 +111,7 @@ handle_idletime_for_event (const ClutterEvent *event)
       event->type == CLUTTER_ENTER ||
       event->type == CLUTTER_LEAVE ||
       event->type == CLUTTER_STAGE_STATE ||
-      event->type == CLUTTER_DESTROY_NOTIFY ||
-      event->type == CLUTTER_CLIENT_MESSAGE)
+      event->type == CLUTTER_DESTROY_NOTIFY)
     return;
 
   core_monitor = meta_idle_monitor_get_core ();