Resolve "[Feature flag] Cleanup async_pipeline_creation_on_push"

What does this MR do and why?

This MR is to cleanup the async_pipeline_creation_on_push feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.

[FF] `async_pipeline_creation_on_push` -- (#558015 - closed)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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

Related to #565422