Refactor DeleteInactiveUsers, fix bug and add tests (#30206) (#30222)
Backport #30206 by wxiaoguang
1. check `IsActive` before calling `IsLastAdminUser`.
2. Fix some comments and error messages.
3. Don't `return err` if "removing file" fails in `DeleteUser`.
4. Remove incorrect `DeleteInactiveEmailAddresses`. Active users could
also have inactive emails, and inactive emails do not support
"olderThan"
5. Add tests