MB-100: Remove unused variable in BackfillTask

Change-Id: I420a74083fcd13738c960af349b47e462bff21d4
Reviewed-on: http://review.couchbase.org/25245
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: xiaoqin ma <maxiaoqin2005@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>