Removing unused generic type from DesignDocument.

Change-Id: I7c13306854a3c986f5b52faabf3b2c792e983c35
Reviewed-on: http://review.couchbase.org/27077
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>