Remove key_set_optimizer_ignored_columns feature flag

What does this MR do and why?

This feature flag was introduced as a safety measure to fix keyset ordering query optimizations.

After testing the fix didn't break anything unexpected in production, this flag is safe to remove

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 #417515 (closed)