Fixed #16833 -- Removed undocumented `mixin` parameter from the...

Fixed #16833 -- Removed undocumented `mixin` parameter from the `Storage.open()` method as this was an undocumented and obscure feature. Thanks to Marty and Russell for sanity-checking.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16824 bcc190cf-cafb-0310-a4f2-bffc1f526a37