gnu: Remove python2 versions of Django packages.

gnu: Remove python2 versions of Django packages.

Python 2 is EOL and not supported by current versions of Django.

* gnu/packages/django.scm (python-django, python-django-tagging)[properties]:
Remove PYTHON2-VARIANT.
* gnu/packages/django.scm (python2-django, python2-django-simple-math-captcha,
python2-pytest-django, python2-django-filter, python2-django-allauth,
python2-django-gravatar2, python2-django-assets, python2-django-jsonfield,
python2-dj-database-url, python2-django-bulk-update,
python2-django-contact-form, python2-django-contrib-comments,
python2-django-overextends, python2-django-redis, python2-django-rq,
python2-django-sortedm2m, python2-django-appconf, python2-django-statici18n,
python2-django-tagging): Remove variables.