Remove unused *_menu_for_rect functions

Unused since 84785951fd ("x11: Remove MetaX11WindowControl").

v2:
* Leave struct _MetaPluginClass::show_window_menu_for_rect, still used
  by gnome-shell.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3154>

src/core/window.c

@@ -5615,15 +5615,6 @@ meta_window_show_menu (MetaWindow         *window,
   meta_compositor_show_window_menu (window->display->compositor, window, menu, x, y);
 }
 
-void
-meta_window_show_menu_for_rect (MetaWindow         *window,
-                                MetaWindowMenuType  menu,
-                                MetaRectangle      *rect)
-{
-  g_return_if_fail (!window->override_redirect);
-  meta_compositor_show_window_menu_for_rect (window->display->compositor, window, menu, rect);
-}
-
 void
 meta_window_shove_titlebar_onscreen (MetaWindow *window)
 {