Remove scan_result_role_action feature flag

What does this MR do and why?

Describe in detail what your merge request does and why.

Remove scan_result_role_action feature flag

• removes the minimum amount of code to remove the feature flag
• remove it from frontend
• remove it from backend
• update tests

Changelog: removed

EE: true

What does this MR not do and why?

To keep this MR smallish (still >500lns changed) and easy to parse, this MR does the minimum to remove the feature flag. It does not do the following for that reason:

• rename the v2 components to not include the v2
• rename the v2 methods to replace the v1
• remove the v1 methods and their tests

Screenshots or screen recordings

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

Page	After
Policy Drawer
Policy Editor - New
Policy Editor - Existing
Search for scan_result_role_action
Search for scanresultroleaction

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 project/group => Security & Compliance => Policies and verify the drawer
3. Select New policy => Scan result policy and verify approvers

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377866 (closed)