Remove `Notify#recipient` in favor of `User#notification_email_for`
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30907 refactors Notify#recipient
so that all the logic is in User#notification_email_for
.
We should just use User#notification_email_for
directly because a lot of times we already have a User
object so doing User.find(recipient_id)
is unnecessary.
The method name is also clearer.