Optimize sent notification keep-around refs

This issue is part of phase 1 of Keep-around ref optimization/attribution (&12961)

app/models/sent_notification.rb:125:    project.repository.keep_around(self.commit_id, source: self.class.name)

These are never created in production. I think this is because the notifications are all for things that keep their own commits. We should stop trying to create keep-around refs for sent notifications.