Remove custom-metrics-api implementation [operator]

[ChristianZaccaria]

WHY

All details are found here: project-codeflare/multi-cluster-app-dispatcher#608

tldr, custom-metrics-apiserver provides a template to implement both the custom and external metrics APIs. However, the implementation of the custom metrics API is not used at all.

The removal of this is also important to reduce RBAC surface.

DONE

Bullet point items for what should be completed:

• Remove everything on custom-metrics-api
• Remove associated resources