Fix RemovedInDjango41Warning about default_app_config

[flaiming]

default_app_config should be removed from https://github.com/django-webpack/django-webpack-loader/search?q=default_app_config to fix Django 4.1 deprecation warning.

RemovedInDjango41Warning: 'webpack_loader' defines default_app_config = 'webpack_loader.apps.WebpackLoaderConfig'. Django now detects this configuration automatically. You can remove default_app_config.

[fjsj]

Closed by #290