Fixed #3094 -- Accelerated deprecation of XMLField, since it hasn't served any...

Fixed #3094 -- Accelerated deprecation of XMLField, since it hasn't served any useful purpose since oldforms. Thanks to PaulM for driving the issue and providing the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15723 bcc190cf-cafb-0310-a4f2-bffc1f526a37