Revert "Support custom authentication backends"

This reverts merge request !334 (merged).

The way custom authentication backends were implemented break too much stuff, or requires workarounds that hide other mistakes. As nothing in AlekSIS uses custom auth backends, and we are currently integratin django-allauth in !413 (merged), I request the feature to be removed.

If someone wants to add a custom authentication backend, they should hook into allauth instead.

@hansegucker Please merge after taking note if you do not have any objective arguments to keep this custom implementation.

Closes #313 (closed)