MB-6231: dont advertise POST on db_doc_req

Which was cut some long time ago.

Change-Id: I19710664e77c255258a48f754eef49e8b17cdde7
Reviewed-on: http://review.couchbase.org/25553
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>