Remove pipeline security dashboard graphql ff

What does this MR do and why?

Related #438235 (closed)

This removes the feature flag definition for pipeline_security_dashboard_graphql as part of the feature flag cleanup. The backend feature flag push is also removed, as are the E2E tests related to the when the feature flag was disabled.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

Pipeline passes