xwayland: Remove undeeded test

xwl_seat_maybe_lock_on_hidden_cursor() checks that the value of
cursor_confinement_window is not NULL, yet there is no code path
that could lead to this.

Remove the test for cursor_confinement_window being set, it's useless.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>