Remove dependency_proxy_manifest_workhorse ff

🍨 What does this MR do and why?

This MR removes the dependency_proxy_manifest_workhorse feature flag that was enabled by default in %14.5.

We also remove the DependencyProxy::PullManifestService as it is no longer used with the removal of this feature flag.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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🍕 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 #344216 (closed)