xwayland/present: Destroy sync_callback in xwl_present_cleanup

xwl_present_cleanup frees the struct xwl_present_window memory,
so if there's a pending callback, we have to destroy it to prevent
use-after-free in xwl_present_sync_callback.

Should fix issue #645.

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
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