Do not reschedule range deletion task when numDeleted < numDocsToRemovePerBatch

Currently, when the last documents in an orphan range have been deleted, the range deletion task gets re-enqueued and re-executed because of this condition. The next execution for the range simply finds no documents, and the range deletion task's document gets deleted.

As a result, if there are thousands of range deletion tasks, it's difficult to monitor the progression because hours/days could pass from the moment an orphaned range is cleared up and the moment its range deletion task's document is deleted.