This reverts commit 7ef6913e71.
We turned on the `ExpirationMailerDontLookTwice` feature flag in prod, and it's
working fine but not clearing the backlog. Since
https://github.com/letsencrypt/boulder/pull/6100 fixed the issue that caused us
to (nearly) stop sending mail when we deployed #6057, this should be safe to
roll forward.
The revert of the revert applied cleanly, except for expiration-mailer/main.go
and `main_test.go`, particularly around the contents `processCerts` (where
`sendToOneRegID` was extracted from) and `sendToOneRegID` itself. So those areas
are good targets for extra attention.