Cleanup multi_pipeline_scan_result_policies feature flag
What does this MR do and why?
Removes multi_pipeline_scan_result_policies which was rolled-out in #409482 (closed)
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.