Remove global_search_error_rate_sli feature flag

What does this MR do and why?

This feature flag has been on for over a month now and is running stable, the feature is completely integrated and we can remove this feature flag.

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 #373800 (closed)