Update label inference to remove orphaned subtypes

What does this MR do and why?

Per #997 (closed)

closes #1003 (closed)

We want to reduce type / subtype label inconsistencies by deleting orphaned subtype labels if type label is changed / removed.

Expectations:

1. when a new type label is added while a previous type label is removed, we want to remove all conflicting subtype labels

2. when a type label is removed, we want to remove all subtype labels associated with that type since they have become orphaned

3. When a subtype label is removed, we do nothing.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (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