Remove useEffect hook validation from create modals

[Arnei]

This hook would call upon the formik validation when the user switched to a different tab in the create modal. This is pointless because A. validation will automatically have been called by formik before then and B. this hook would only trigger after the tab has changed, which would be too late anyway.

[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-1486

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-1486

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.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/1486/2025-12-18_14-54-04/ .
It might take a few minutes for it to become available.