Stop asking contributors to assign GDK category labels

What does this merge request do and why?

After gitlab-org/quality/triage-ops!2284 (merged) last year and gitlab-org/quality/triage-ops!2918 (merged) last week, we can stop asking contributors to set the labels they tick off in issues and merge requests because that now happens automatically on both.

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.