Remove gitlab-metrics-exporter image builds

What does this MR do?

Remove gitlab-metrics-exporter image builds. This system is being retired. See gitlab-org/gitlab-metrics-exporter#31 Not user-facing because it was never in active use. So no docs changes required either.

• release-tools change: gitlab-org/release-tools!2491 (merged)
• Omnibus change: gitlab-org/omnibus-gitlab!7030 (merged)

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io