Remove _design listing from couchbase.rst

Change-Id: I2b46046edbf975d85fddfc597f3a6abf15f713b3
Reviewed-on: http://review.couchbase.org/26768
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>