Do not infer a section for the unlisted stage and its groups

Context

Follow-up of gitlab-com/www-gitlab-com!132396 (merged), where we created an unlisted_stage stage and some groups underneath that stage.

Closes #1456 (closed).

What does this MR do and why?

Our label inference system tries to guess a section from either a stage or a group.

This will end up using the unlisted section, which does not exist!

This MR ensures that we don't return the unlisted section when doing label inference.

Expected impact & dry-runs

Should fix errors seen in #1456 (closed).

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review