Remove `delete_scan_result_policies_by_project_id` feature flag

What does this MR do and why?

Removes the delete_scan_result_policies_by_project_id feature flag introduced in %16.2 by !119763 (merged) and rolled out by #415912 (closed). The flag is globally enabled on production.

MR acceptance checklist

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

Related to #415912 (closed)