clutter: Remove DELETE event, signal and vfunc

The delete event was used for signalling the close button was clicked on
clutter windows. Being a compositor we should never see these, unless
we're running nested. Remove the plumbing of the DELETE event and just
directly call meta_quit() when we see it, if we're running nested.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1364

src/core/events.c

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