window-private: Actually remove frame instance

As per GNOME/mutter!3254 MetaFrame is part of MetaWindowX11Private, but
MetaWindow has still the frame field.

Drop it since it's unused now and can lead to confusion

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3784>

src/core/window-private.h

@@ -216,8 +216,6 @@ struct _MetaWindow
   MetaLogicalMonitor *highest_scale_monitor;
   MetaWorkspace *workspace;
   MetaWindowClientType client_type;
-  /* may be NULL! not all windows get decorated */
-  MetaFrame *frame;
   int depth;
   char *desc; /* used in debug spew */
   char *title;