Remove ConcurrencyLimit::WorkersMap class

What does this MR do and why?

We removed the usage of ConcurrencyLimit::WorkersMap in !211285 (merged) as part of the feature flag removal.

We're removing the definition here and the rest of the references.

References

Part of cleanup https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/267

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