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.
question: do you think we should name it something else?  I tried to think of a name that hinted at what it does, but I couldn't.  Something like ReportUnknownEmailsPasswordResetForm but a lot shorter.  Any ideas?