[Feature flag]: Clean up feature flag security_policies_kev_filter
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<!--Title suggestion: [Feature flag] Cleanup <feature-flag-name>-->
## Summary
This issue is to cleanup the `<security_policies_kev_filter>` 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: @arfedoro @imam_h
- PM: @alan
## Stakeholders
<!--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-->
## 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-->
- [ ] 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](https://docs.gitlab.com/ee/development/feature_flags/#changelog).
- [ ] 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>`
- [ ] 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 --pre --staging --staging-ref --production`
- [ ] Close this rollout issue.
<!--Uncomment the appropriate type label
/label ~"type::feature" ~"feature::addition"
/label ~"type::maintenance"
/label ~"type::bug"-->
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