CBQE-0: remove unused variable src_cluster_name from _link_clusters function

Change-Id: I6cf5b802a3f33cb2a00d39471cae05563729aaca
Reviewed-on: http://review.couchbase.org/41077
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>