Revert https://code.djangoproject.com/ticket/19758

django-authtools makes email addresses unique.

The bug #19758 tries not to leak emails through password reset because
only usernames are unique in Django's default user model.

django-authtools leaks email addresses through the registration form.
In the case of django-authtools not warning the user doesn't add any
security, and worsen user experience.
`is not None`?