Don't support design documents updates via _bulk_docs.

Change-Id: I4336080ff941a9266b86adbef28d2bd290dd2035
Reviewed-on: http://review.couchbase.org/40035
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>