x11: Remove hide-titlebar-when-maximized support

It's a UI pattern that has been superseded by client-side decorations,
apps that used to set the hint have generally moved on to headerbars.
Given that and the limitation to server-side decorated X11 windows,
GTK4 removed the client-side API for setting the hint, it's time to
follow suite and retire the feature.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/221

src/core/window-private.h

@@ -189,7 +189,6 @@ struct _MetaWindow
   char *gtk_app_menu_object_path;
   char *gtk_menubar_object_path;
 
-  int hide_titlebar_when_maximized;
   int net_wm_pid;
 
   Window xtransient_for;