Stop including core.h where it's not used

It provides glue for window frame controls and internal window
management, and that is only relevant for a couple of places in src/ui/.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/854

src/x11/window-x11.c

@@ -36,7 +36,6 @@
 #include "backends/meta-logical-monitor.h"
 #include "backends/x11/meta-backend-x11.h"
 #include "core/boxes-private.h"
-#include "core/core.h"
 #include "core/frame.h"
 #include "core/meta-workspace-manager-private.h"
 #include "core/window-private.h"