remove not needed last_process from ns_couchdb_sup

Change-Id: Iddcba70ec1decee4bbfc9d33ee8c998f1cc5512d
Reviewed-on: http://review.couchbase.org/43815
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>