[Feature flag] Cleanup `single_file_file_by_file`

Summary

This issue is to cleanup the single_file_file_by_file feature flag, since the feature flag has been enabled by default for quite a while in production.

We will completely remove the definition for the flag, and the small remaining code that refers to it.

Owners

• Team: groupcode review
• Most appropriate slack channel to reach out to: #g_create_source-code-review-fe
• Best individual to reach out to: @thomasrandolph
• PM: @phikai

Stakeholders

Expectations

What might happen if this goes wrong?

Cleaning up the feature flag

• Specify in the issue description if this feature will be removed completely or will be productized as part of the Feature Flag cleanup
• 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 --ops --pre --staging --staging-ref --production
• Close this rollout issue.