Modified [7112] to make things behave more in line with Python subclassing when subclassing ModelForms.

Meta can now be subclassed and changes on the child model affect the fields
list. Also removed a block of error checking, since it's harder to mess up in
unexpected ways now (e.g. you can't change the model and get the entirely wrong
fields list), so it was a level of overkill.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7115 bcc190cf-cafb-0310-a4f2-bffc1f526a37