Remove backfill code and unused method.

* remove the backfill code which has caused some grief
* remove unused method

We now log gaps that exceed the heartbeat threshold but do not backfill with null/empty rows.

Backfilling gaps has caused data loss in the rare situation that we get data into the persister out of order. The intentions of this backfilling are lost in the mists of time and the approach seems dubious as it might casuse data to be lost.

The _old_aggregate_base_rate method was kept around to do testing and to triage a bug, it is no longer needed.