Remove security_policies_severity_customize feature flag

What does this MR do and why?

Removes the security_policies_severity_customize feature flag introduced in 18.9 (default_enabled: true). The flag has been running in production without issues and is ready for cleanup in 19.0.

The flag gated the severity override action in vulnerability management policies — both the backend auto-override pipeline worker/service and the frontend policy editor UI components. All code behind the flag is now unconditional.

References

• Cleanup issue: #596540
• Original feature issue: gitlab-org#15839 (closed)
• Introduced by: !222151 (merged)
• Rollout issue: #588540 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Ensure no references to security_policies_severity_customize remain: grep -r security_policies_severity_customize ee/ --include="*.rb" --include="*.vue" --include="*.js" --include="*.yml" → should return nothing
2. Navigate to a project's security policies page
3. Create a vulnerability management policy — the severity override action type should be available without any feature flag configuration

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.