Remove dead endpoint EE::Clusters::ClustersController#metrics

What does this MR do?

During work at #208224 (closed) I've noticed possibly dead code

It looks like #metrics endpoint in ee/app/controllers/ee/clusters/clusters_controller.rb is no longer in use. From what I managed to track from repo history it was used to fetch additional metrics. It is passed to vue compenent here b4573bd8 and it is later used by 871bf96c but it not used any more, so instead of moving it to core, we can remove it for codebase.

Endpoint was added gitlab-foss!17139 (5db198fd)

Issue: #225869 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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• Style guides
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