Stop reading authzVersion to decide which users collection to read.

In 2.6, to support upgrading from 2.4, the auth system would read user documents from one of several places based on the current "authzVersion". In 2.8, user documents are always read from the same place, because the auth upgrade is in-place. We should remove support for this decision, as it simplifies concurrency control code and will reduce the risk of introducing hidden errors during upgrade.