prefs: Allow no-op listener removal

Allow removing a prefs handler that was already removed. This allows us
to remove prefs from the dispose function without having to keep track
of it in every place.

src/core/prefs.c

@@ -860,8 +860,6 @@ meta_prefs_remove_listener (MetaPrefsChangedFunc func,
 
       tmp = tmp->next;
     }
-
-  meta_bug ("Did not find listener to remove\n");
 }
 
 static void