Remove graphql_pipeline_header feature flag

What does this MR do?

This removes the graphql_pipeline_header feature flag as the rollout of the aforementioned flag was successful

Rollout issue: #254235 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content