Remove unused BackfillCallback class
Remove unused BackfillCallback class

Change-Id: I3db67622e097160a135dee22eab21f8cbd22384a
Reviewed-on: http://review.couchbase.org/48954
Reviewed-by: Manu Dhundi <158873d90a7ef40f3637a222b7329c09d0222554@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

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