window-actor: Remove left-over parent field

The commit

commit 60f7ff3a69
Author: Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
Date:   Fri Dec 21 18:12:49 2018 -0200

    window-actor: Turn into a derivable class

made the previous instance struct a instance private struct, but didn't
remove the parent field. Since it's unused, there is no point in keeping
it around, so lets drop it.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/658

src/compositor/meta-window-actor.c

@@ -51,8 +51,6 @@ typedef enum
 
 typedef struct _MetaWindowActorPrivate
 {
-  ClutterActor           parent;
-
   MetaWindow *window;
   MetaCompositor *compositor;