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.c

@@ -117,6 +117,7 @@ static void unminimize_window_and_all_transient_parents (MetaWindow *window);
 
 static void meta_window_propagate_focus_appearance (MetaWindow *window,
                                                     gboolean    focused);
+static void meta_window_update_icon_now (MetaWindow *window);
 
 /* Idle handlers for the three queues (run with meta_later_add()). The
  * "data" parameter in each case will be a GINT_TO_POINTER of the
@@ -4822,7 +4823,7 @@ redraw_icon (MetaWindow *window)
     meta_ui_queue_frame_draw (window->screen->ui, window->frame->xwindow);
 }
 
-void
+static void
 meta_window_update_icon_now (MetaWindow *window)
 {
   GdkPixbuf *icon;