[Cleanup] Packet validator will only call DCP func on DCP conn

We don't need to add code to gracefully deal with DCP commands
being receive on non-dcp connections inside ep-engine.

Change-Id: I04e7c080484cda7aecd07aaf84a5b865dbcaf38d
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/140964
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: trondn