Remove Tap privilege argument

kv_engine would like to remove the Tap priviledge as it is no longer required.
However on removing it from kv_engine it was uncovered that ns_server is passing the Tap privilege.

The request is for ns_server be updated to no longer pass the Tap privilege
Then kv_engine can make a subsequent change to remove its definition.

(FYI - all TAP commands have been changed to require no privilege - and will return NOT_SUPPORTED if a TAP command is invoked - see http://review.couchbase.org/#/c/81488/)