Remove feature flag invalid_scan_result_policy_prevents_merge

What does this MR do and why?

This MR removes feature flag invalid_scan_result_policy_prevents_merge

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 #405023 (closed)