Remove instructions for running transient bug tests

What does this MR do and why?

Removes obsolete instructions for running transient bug tests. The jobs for those tests were removed in gitlab-org/quality/pipeline-common!276 (merged). The remaining tests are being removed in gitlab!176058 (merged)

See gitlab-org/quality/quality-engineering/team-tasks#3273 (closed)

MR acceptance checklist

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• Commits include Changelog: trailer
• I have evaluated the MR acceptance checklist for this MR.