Remove uncached_mr_attention_requests_count feature flag

What does this MR do and why?

This MR removes uncached_mr_attention_requests_count feature flag that we no longer need.

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 #366442 (closed)