killed unused ns_cluster:add_node function

Change-Id: I2538b43a3fe432a6c4c7bbd5e66fae908a8cc113
Reviewed-on: http://review.couchbase.org/42865
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>