Simplify maintenance of persistent_count on ack and purge:

* Don't parameterise remove_queue_entries/5 with a fold function, there's
  only one call site.
* Get remove_queue_entries/4 / purge_betas_and_deltas/3 to update persistent
  count directly, not removed counts by store.
* Update persistent count in one place in remove_pending_ack/2, not many
  places in ack/2.
* Remove a couple of no-longer needed helpers.
Maybe a tad clearer?
Merge bug26335