Remove kw.forms.is_valid_kw_civilid_checksum before the 6.0 release

This is a reminder to remove `kw.forms.is_valid_kw_civilid_checksum` before the 6.0 release. I don't know if we'll do a 5.1 release, which is why I'm not removing this deprecated function now.

https://github.com/django/django-localflavor/blob/master/localflavor/kw/forms.py#L23-L32