Remove unused VulnerabilityReportsComparer class

What does this MR do and why?

The VulnerabilityReportsComparer class was removed in Remove security widget findings FF (!182541 - merged) • Rushik Subba • 17.10 and replaced with Gitlab::Ci::Reports::Security::SecurityFindingsReportsComparer, however, there are still tests and other references to this unused class, which this MR removes.

References

[Feature flag] Enable migrate_mr_security_widge... (#515281 - closed) • Rushik Subba • 17.10