remove upr_sup:nuke call from janitor_agent:terminate

because:
- it is not executed anyways if janitor_agent is brutally killed by sup
- it will be executed before very first list_vbuckets call in newly
  started janitor_agent

Change-Id: I56b17732a51a244b8392d77e88306892d4ead744
Reviewed-on: http://review.couchbase.org/38532
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>