Remove expired ports:

2020-07-15 www/py-django-subdomains: No longer maintained, not compatible with Django 2.0+
2020-07-15 www/py-django-datetime-widget: No longer maintained, not compatible with Django 2.1
2020-07-15 www/py-django-openid-auth: No longer maintained, not compatible with Django 2.0+, please use www/py-django-allauth instead
2020-07-15 www/py-django-overextends: No longer maintained, not compatible with Django 2.0+