Fixed #6991 -- Removed some redundant user.is_authenticated() calls in various...

Fixed #6991 -- Removed some redundant user.is_authenticated() calls in various places. Thanks, alexkoshelev, Liang Feng and Ivan Sagalaev

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12142 bcc190cf-cafb-0310-a4f2-bffc1f526a37