backends/x11: Do not reload keymap on new keyboard notifications

XkbNewKeyboardNotify informs the client that there is a new keyboard
driving the VCK. It is essentially meant to notify that the keyboard
possibly has a different range of HW keycodes and/or a different
geometry.

But the translation of those keycodes remain the same, and we don't
do range checks or geometry checks (beyond using KEY_GRAVE as "key
under Esc", but that is hardly one). It seems we can avoid the
busywork that is releasing all our passive grabs, reloading the keymap
and regenerating the keycombos and restoring the passive grabs.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/398

src/backends/x11/meta-backend-x11.c

@@ -357,7 +357,6 @@ handle_host_xevent (MetaBackend *backend,
         {
           switch (xkb_ev->any.xkb_type)
             {
-            case XkbNewKeyboardNotify:
             case XkbMapNotify:
               keymap_changed (backend);
               break;