Remove deprecate_scan_result_policies flag definition

What does this MR do and why?

This is part 3 of a 3-MR stack splitting !251778 (closed). It targets MR 2's branch (!252701 (merged)) and merges last, once MR 1 (!252700 (merged)) and MR 2 are merged, since it deletes the deprecate_scan_result_policies flag definition — only safe after all usages elsewhere are gone.

It collapses the last remaining Feature.enabled?(:deprecate_scan_result_policies, ...) branch and then removes the flag itself. The for_scan_result_policy_read fallback for reads without an approval_policy_rule_id is kept; the table stays load-bearing until the drop tracked in the table-drop work item below.

Changes:

• Collapses the FF branch in Security::ScanResultPolicies::LicenseViolationChecker, keeping the for_scan_result_policy_read fallback.
• Deletes the flag definition ee/config/feature_flags/wip/deprecate_scan_result_policies.yml.
• Deletes the shared spec context 'with deprecate_scan_result_policies flag disabled' and removes the remaining FF-disabled contexts/comments from 6 spec files.

References

• #510282 (closed)
• #617802

Zero-downtime deployment safety

The flag is explicitly enabled on GitLab.com, so old-version nodes still evaluating Feature.enabled? (before this MR's deploy completes) take the same enabled path during a rolling deploy; no migrations or schema changes are involved.

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