dbwrap_watch: Simplify struct dbwrap_watched_watch_state

With the wakeup messages changed, we don't need to store what used to
be the old message in the struct dbwrap_watched_watch_state
anymore. We still need to store the key that is watched for removal of
our watch entry in the request destructor

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
