Remove sidekiq_concurrency_limit_middleware_v2 feature flag

What does this MR do and why?

Remove sidekiq_concurrency_limit_middleware_v2 feature flag

This feature flag is a derisk feature flag meant to derisk changes when we roll out changes to refactor concurrency limit middleware !201922 (merged).

The sidekiq_concurrency_limit_middleware_v2 feature flag was used to direct code path to ConcurrencyLimit::MiddlewareV2 class.

References

#565604

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