Remove Bucket Flush from Memcached Audit Event

We currently audit a bucket flush from an ns_server event and a memcached event.
However the memcached event is only generated for memcached buckets and not for couchbase buckets. This is confusing to the user.

The flush command returns not supported for the ep bucket engine (ns_server uses a different mechanism - to flush the bucket)
See http://src.couchbase.org/source/xref/vulcan/kv_engine/engines/ep/src/ep_engine.cc#2331

Therefore we should remove the flush bucket audit event from memcached and just use the ns_server flush bucket audit event.