Remove dead code

[Arnei]

Removes left over code from modernizing redux
(which is still not completely done).

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/646/2024-06-05_13-23-54/ .
It might take a few minutes for it to become available.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-646

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-646

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[ziegenberg]

Clicked through the relevant parts of the UI, saw no additional errors or warnings and it still works.