refactor(django): remove deprecated `django` plugin

BREAKING CHANGE: the `django` plugin was deprecated in 2021-09-22. With
this change it has now been removed altogether. Zsh already provides
built-in completion for Django commands.

plugins/django/README.md

-# Django plugin
-
-This plugin adds completion for the [Django Project](https://www.djangoproject.com/) commands
-(`manage.py`, `django-admin`, ...).
-
-## Deprecation (2021-09-22)
-
-The plugin used to provide completion for `./manage.py` and `django-admin`, but Zsh already provides
-a better, more extensive completion for those, so this plugin is no longer needed.
-
-Right now a warning message is shown, but in the near future the plugin will stop working altogether.
-So you can remove it from your plugins and you'll automatically start using Zsh's django completion.