Remove unused BackfillDiskLoad taskid
Remove unused BackfillDiskLoad taskid

This task no longer exists, but the TaskId was not removed. Cleaning
it up.

Change-Id: I216e513cdfa62cca4e63d181241e0a7e69dbae61
Reviewed-on: http://review.couchbase.org/118057
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine