clutter: Drop ClutterInputDevice private tool maintenance API

This is just used in the native backend (with the X11 going its own
way). Just keep a HT of tools there, and drop this API.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

src/backends/native/meta-seat-impl.h

@@ -62,6 +62,7 @@ struct _MetaSeatImpl
   GRWLock state_lock;
 
   GSList *devices;
+  GHashTable *tools;
 
   ClutterInputDevice *core_pointer;
   ClutterInputDevice *core_keyboard;