Remove feature flag `cache_client_with_metrics`

Vasilii Iakliushinrequested to merge
remove_cache_client_with_metrics_ff into master

What does this MR do and why?

Contributes to #392622 (closed)

Use Redis client with metrics support for API::Files#content_sha.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Vasilii Iakliushin

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