Optimize query for issue neighbors
What does this MR do and why?
Remove optimized_issue_neighbor_queries feature flag so that the code
will use the new query which is already tested in production for more than a month.
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 #345921 (closed)
Edited by Heinrich Lee Yu