MB-5457 Remove duplicate memory threshold check on backfill.
MB-5457 Remove duplicate memory threshold check on backfill.

Memory threshold check on backfill is actually performed right
before scanning a vbucket's database.

Change-Id: I73c80088052f8cbf8e1399cc80f55caeff29d29e
Reviewed-on: http://review.couchbase.org/17128
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

membase/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,daverigby/kv_engine,abhinavdangeti/ep-engine,daverigby/ep-engine,daverigby/kv_engine,couchbase/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,hisundar/ep-engine,couchbase/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,membase/ep-engine,membase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/ep-engine,owendCB/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,hisundar/ep-engine