Remove ns_log related stuff from ns_cluster_membership.

Because it's not used.

Change-Id: I9e66463980b6e0f4521e3e42b0654ef6bc7bab5f
Reviewed-on: http://review.couchbase.org/32389
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>