Removed the ado_mssql database backend.

It has not been maintained, contains bugs, and improved versions are available
externally(e.g. django-mssql and django-pyodbc at Google code).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7364 bcc190cf-cafb-0310-a4f2-bffc1f526a37