The "notifications" query currently uses offset paging for no apparent reason (just a legacy issue?), so some of the paging code is only reachable internally.
- Stop it from using offset paging, since modern cursor paging is fine here (and Feed has used cursor paging for a long time).
- Fix the non-offset paging to work like Feed.
- Remove a couple of stub methods with no callsites after cursor refactoring.