Remove all code related to clusters/integrations

What does this MR do and why?

This was used for Prometheus integration through the certificate-based cluster: Add (BYO) Prometheus as cluster integration (!55244 - merged). This integration was later on removed because we will be deprecating certificate-based clusters: Monitor: Metrics Deprecation (#346541 - closed).

The related UI has also been remove completely:

• Hide Cluster Integrations tab when remove_monit... (!120600 - merged)
• Delete cluster integration UI (!122043 - merged).

So we can now remove this unused backend code.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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How to set up and validate locally

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Related to #434609 (closed)