Update devops labels spec and test fixture

What does this MR do and why?

Address the failing devops spec mentioned in #1175 (closed)

closes mentioned in #1175 (closed)

3 changes were made in this MR:

1. Fix the failing spec by updating the expected outcome of comment_for_intelligent_stage_and_group_labels_inference when working with the groupglobal search group. See issue description for reason why this test started failing.
2. Remove fixtures in the spec/fixtures/scheduled folder which has become out of date for a while. The devops_label_spec doesn't actually run against those fixtures unless we only run the #comment_for_intelligent_stage_and_group_labels_inference, which was very confusing at first during my debugging process. It produced different results when I run bundle exec rspec spec/lib/devops_labels_spec.rb versus bundle exec rspec spec/lib/devops_labels_spec.rb:745 due to different fixtures being applied.
3. Removed references to the spec/fixtures/scheduled fixtures in the devops label spec, and added stubbing for the categories endpoint which wasn't applied to every spec before.

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