[Feature flag] Cleanup `security_policies_variables_precedence`
## Summary
This issue is to cleanup the `security_policies_variables_precedence` feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.
## Owners
- Team: ~"group::security policies"
- Most appropriate slack channel to reach out to: `#g_govern_security_policies`
- Best individual to reach out to: @mcavoj
- PM: @g.hickman
## Stakeholders
## Expectations
### What might happen if this goes wrong?
### Cleaning up the feature flag
- [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 `security_policies_variables_precedence` 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 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 security_policies_variables_precedence --dev --pre --staging --staging-ref --production`
- [x] Close this rollout issue.
issue