Ignore subtitle tracks in select-tracks WHO

[snoesberger]

fixes #5817

Subtitle tracks interfere with the proper functioning of the select-track WHO. Therefore we have to exclude them from the selection process.

Your pull request should…

• have a concise title
• close an accompanying issue if one exists
• be against the correct branch
• [ ] include migration scripts and documentation, if appropriate
• pass automated tests
• have a clean commit history
• have proper commit messages (title and body) for all commits

[github-actions]

Hi @snoesberger
Thank you for contributing to Opencast.
We noticed that you have not yet filed an Individual Contributor License Agreement. Doing that (once) helps us to ensure that Opencast stays free for all. If you make your contribution on behalf of an institution, you might also want to file a Corporate Contributor License Agreement (giving you as individual contributor a bit more security as well). It can take a while for this bot to find out about new filings, so if you just filed one or both of the above do not worry about this message!
Please let us know if you have any questions regarding the CLA.

[Arnei]

Makes sense to me and works