Deprecated csrf_response_exempt and csrf_view_exempt decorators

With the removal of CsrfResponseMiddleware, csrf_response_exempt serves no
purposes, and csrf_exempt and csrf_view_exempt perform the same function.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15956 bcc190cf-cafb-0310-a4f2-bffc1f526a37