Remove prioritization of changesets without any diffstat

Created by: eseliger

We did this, because in a previous migration we would lose those diffstats, and we wanted those changesets to be back in shape as soon as possible. Given that we now copy it over from the changeset_spec, this is never not known, but computable. We cannot make it non-nullable in GQL and the DB currently, though, because imported changesets that were imported when the changeset was already closed don't have a diffstat either. This also fixes us prioritizing those always on repo-updater startup.

Closes https://github.com/sourcegraph/sourcegraph/issues/16604