Remove async_mr_close_issue feature flag

What does this MR do and why?

Remove the async_mr_close_issue feature flag to enable the async closure of issue by default.

It's been enabled on production for a week and it's performing fine and no bugs were reported.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361320 (closed)