Remove derisk feature flag

What does this MR do and why?

Remove derisk feature flag

• remove unused code
• clean up tests
• remove feature flag

Changelog: removed

EE: true

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	Screenshot
Save new policy
Error with not enough approvers
Error with invalid branches

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Navigate to a new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
3. Verify the user is navigated to the newly created security policy project with an MR
4. Navigate to the new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
5. Verify the user is navigated to the security policy project with an MR
6. Navigate to the new project/group => Secure => Policies => Edit policy project => Remove the linked project
7. Navigate to the new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
8. Verify the error appears and a user can still take actions on the page
9. Repeat the steps for every policy type
10. Enable the ff echo "Feature.enable(:security_policies_project_background_worker)" | rails c
11. Verify the steps above work for every policy type

Related to #482525 (closed)