Remove APICredential-based machinery

[sergei-maertens]

We can rely 100% on zgw-consumers instead of having this mixed approach based on a vanilla gemma-zds-client implementation.

zgw-consumers is richer and provides django support. The core mechanisms (get_auth etc.) should be adapted to build on top of zgw-consumers.

[annashamray]

Open Notificaties which heavily relies on APICredential should not be forgotten here