core: Remove useless meta_invalidate_default_icons implementation

This code was supposed to refresh our default icons when the theme
changed, but it actually was a no-op, since the default icons are cached
in a static variable in MetaUI.

I'm not sure the fact that the fallback icons don't update when the
theme changes is an important enough use case to keep working, but I'm
keeping the skeleton function there in case somebody wants to actually
fix it properly.

src/core/window-private.h

@@ -632,8 +632,6 @@ void meta_window_stack_just_below (MetaWindow *window,
 void meta_window_set_user_time (MetaWindow *window,
                                 guint32     timestamp);
 
-void meta_window_update_icon_now (MetaWindow *window);
-
 void meta_window_update_for_monitors_changed (MetaWindow *window);
 void meta_window_update_on_all_workspaces (MetaWindow *window);