MB-9096: killed unused handling of old-style mb_master heartbeats

Change-Id: Ic9f7cad31f4138df0556dd1d82acfafa5f316eef
Reviewed-on: http://review.couchbase.org/29323
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>