killed old concurrency throttle

Change-Id: Ia753168cf234832281fe1c93d02fec9debfad429
Reviewed-on: http://review.couchbase.org/40359
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>