Enable async_after_approval feature flag
What does this MR do and why?
This removes the async_after_approval feature flag and keep the new code.
Old code on ApprovalService has been removed as well since they won't be used anymore.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #368098 (closed)
Edited by Patrick Bajao