MB-8665: killed unused and broken mb_master_events

mb_master_events was broken because it would only fire on new master
or old master nodes. And it was previously used to try to refresh pool
details for update of recently killed stopRebalanceIsSafe field.

Change-Id: I97385195a405a40c3b5f24ece9f67dd6bd7c6485
Reviewed-on: http://review.couchbase.org/28885
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>