Remove duplicate memory constraint checks on backfill.
Remove duplicate memory constraint checks on backfill.

Memory constraints are already checked right before scheduling
a single vbucket backfill task.

Change-Id: I990357358b4d7e3837a32c726bb5012710731be0
Reviewed-on: http://review.couchbase.org/13343
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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