Remove unused DjangoContext creation

[sobolevn]

It was not used at all. If we ever need this code back (only if the plugin is updated to support new features, while I don't use drf anymore and do not plan to work on this), we can use this compat code:

        plugin_config = DjangoPluginConfig(options.config_file)

        # XXX: this is a compat code for older `DjangoContext` API.
        try:
            # First, we try the old signature. The new API would fail with `RuntimeError`.
            # While, the old one would just work
            self.django_context = DjangoContext(plugin_config.django_settings_module)
        except RuntimeError:
            # New API:
            self.django_context = DjangoContext(plugin_config)

I've also updated typeddjango/django-stubs#1557 to have all the compat code we need.

Closes #432

[intgr]

Thanks!