Remove contribution_analytics_optimized_base_query FF

What does this MR do and why?

This MR removes the contribution_analytics_optimized_base_query feature flag. The flag was enabled by default in 15.4: !98028 (merged)

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 #367090 (closed)