[1.2.X] Fixed #8342 -- Removed code from the admin that assumed that you can't...

[1.2.X] Fixed #8342 -- Removed code from the admin that assumed that you can't login with an email address (nixed by r12634).

Backport from trunk (r14769).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14773 bcc190cf-cafb-0310-a4f2-bffc1f526a37