Removed unused code from admindocs app now that it supports reversing URLs.

As a consequence the undocumented `ADMIN_SITE_ROOT_URL` setting isn't used anymore.

Refs r8718 and r16857.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16898 bcc190cf-cafb-0310-a4f2-bffc1f526a37