Clean up async_mirror_branch_creation feature flag

What does this MR do and why?

Clean up async_mirror_branch_creation feature flag

It has been default enabled since 19.3. This change keeps the enabled behaviour and removes the feature flag

Resolves #600900 (closed)

Changelog: other

References

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 #600900 (closed)