Remove unused tap and dcp code in default_engine

Change-Id: Ife41e5d0bd38d8721c234e01d42fb9eecf0b918c
Reviewed-on: http://review.couchbase.org/43174
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>