[auth][cleanup] fully delete user tuples

[axiomofjoy]

We currently have an optional deleted_at timestamp field in the users table of the database that we add a value to when a user is deleted. Since we also have a unique constraint on email, this means that once a user has deleted their account, the email is no longer usable in Phoenix. This problem is exacerbated with our OAuth2 integration, since it means that a user has no way to log with their IDP if they have already used the corresponding email to create a local account.

We need to remove the deleted_at column and actually delete user tuples to resolve this issue. For reference, we want to undo the changes that were made in this PR.