Remove gitlab-runner from asdf-combine

What does this merge request do and why?

This MR is inspired by !3664 (closed) and tries to make an even smaller iteration to remove the gitlab-runner/.tool-versions file from the GDK .tool-versions file.

If this works, we can remove all asdf-combine logic (CI etc.)

See #1635 (closed)

How to set up and validate locally

1. Run gdk config set gitlab_runner.enabled true
2. Run GDK_SELF_UPDATE=0 gdk update
3. Go to gitlab-runner
4. See that go and yd are available

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.