Remove "completed" column from servers table

[Arnei]

The column "jobs completed" had already been removed from the servers table in the old admin ui. The backend is not even offering any data for this column anymore.
Therefore, this patch gets rid of it.

Fixes #1155.

How to test this

Can be tested as usual, but may require clearing browser cache. Check that nothing of this column remains.

[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-1177

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-1177

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/1177/2025-04-02_12-53-34/ .
It might take a few minutes for it to become available.