[Feature flag] Cleanup `ci_remove_legacy_predefined_variables`

Summary

This issue is to cleanup the ci_remove_legacy_predefined_variables feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.

The flag was enabled to default ON in the roll out issue: #404533 (closed)

This FF is planned for removal in %16.1

Owners

• Team: Pipeline Authoring
• Most appropriate slack channel to reach out to: #g_pipeline_authoring
• Best individual to reach out to: @lma-git
• PM: @dhershkovitch

Expectations

What might happen if this goes wrong?

If something goes wrong, the clean up MR (!121582 (merged)) can be rolled back.

Cleaning up the feature flag

• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• If not already done, clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete <feature-flag-name> --dev
  • /chatops run feature delete <feature-flag-name> --staging
  • /chatops run feature delete <feature-flag-name>
• Close this rollout issue.