Remove deprecated EE::Ci::Pipeline#with_legacy_security_reports method

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Proposal

During Spike: Where do we use Dependency Scanning repo... (#394728 - closed), it was discovered that this method is no longer used anywhere in the codebase and can be removed safely.

Implementation plan

1. Grep the codebase for references to with_legacy_security_reports
2. If no references exist, remove the method from EE::Ci::Pipeline

This method is no longer used anywhere in the codebase, so it can be safely removed.