gobject: drop ref/unref from g_object_freeze_notify()

This was done since the beginning (commit e773d7dba6 ('fixed dealing
with collection/lcopy of NULL values.'). But it's not clear, why we
would need to take a reference on the calling object.

Freeze does not emit any signals/callbacks and does not call back to the
user. It just sets up some internal state.

This doesn't require to take a reference. The caller must hold a valid
reference to being with, but if that's given, there is no need to
acquire another reference.

gobject/gobject.c

@@ -1443,9 +1443,7 @@ g_object_freeze_notify (GObject *object)
     }
 #endif
 
-  g_object_ref (object);
   g_object_notify_queue_freeze (object, FALSE);
-  g_object_unref (object);
 }
 
 static inline void