clutter: Make clutter_event_get_source() only about crossing events

Avoid peeking in the stage for loosely related actors, since the same
event could be handled by different actors across the picking stack.

This getter is also unused, so there's wiggle room here for changes.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3153>

clutter/clutter/clutter-event.c

@@ -514,8 +514,7 @@ clutter_event_get_source (const ClutterEvent *event)
       event->type == CLUTTER_LEAVE)
     return event->crossing.source;
 
-  return clutter_stage_get_event_actor (clutter_event_get_stage (event),
-                                        event);
+  return NULL;
 }
 
 /**