[Feature flag] Cleanup policies_always_override_project_ci
<!-- Title suggestion: [Feature flag] Cleanup <feature-flag-name> -->
## Summary
This issue is to cleanup the `policies_always_override_project_ci` feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.
<!-- Short description of what the feature is about and link to relevant other issues. Ensure to note if the feature will be removed completely or will be productized-->
## Owners
- Team: Security Policies
- Most appropriate slack channel to reach out to: `#g_srm_security_policies`
- Best individual to reach out to: `@mcavoj`
- PM: `@g.hickman`
## Stakeholders
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Are there any other stages or teams involved that need to be kept in the loop?
- Name of a PM
- The Support Team
- The Delivery Team
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## Expectations
### What might happen if this goes wrong?
<!-- Any MRs that need to be rolled back? Communication that needs to happen? What are some things you can think of that could go wrong - data loss or broken pages? -->
### Cleaning up the feature flag
<!-- The checklist here is to help stakeholders keep track of the feature flag status -->
- [x] Specify in the issue description if this feature will be removed completely or will be productized as part of the Feature Flag cleanup
- [x] Create a merge request to remove `policies_always_override_project_ci` feature flag. Ask for review and merge it.
- [x] Remove all references to the feature flag from the codebase.
- [x] Remove the YAML definitions for the feature from the repository.
- [x] Create [a changelog entry](https://docs.gitlab.com/ee/development/feature_flags/#changelog).
- [x] Ensure that the cleanup MR has been deployed to both production and canary.
If the merge request was deployed before [the code cutoff](https://about.gitlab.com/handbook/engineering/releases/#self-managed-releases-1),
the feature can be officially announced in a release blog post.
- [ ] `/chatops run auto_deploy status <merge-commit-of-cleanup-mr>`
- [x] Close [the feature issue](https://gitlab.com/gitlab-org/gitlab/-/issues/512123) to indicate the feature will be released in the current milestone.
- [x] If not already done, clean up the feature flag from all environments by running these chatops command in `#production` channel: `/chatops run feature delete policies_always_override_project_ci --dev --pre --staging --staging-ref --production`
- [x] Close this rollout issue.
<!-- Uncomment the appropriate type label
/label ~"type::feature" ~"feature::addition"
/label ~"type::maintenance"
/label ~"type::bug"
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issue