Outdated GPU monitoring info in README

[eero-t]

Monitoring README claims to provide GPU usage through cAdvisor "accelerator" metrics: https://github.com/kubeflow/training-operator/blob/master/docs/monitoring/README.md

However, those have been removed from latest cAdvisor release: https://github.com/google/cadvisor/releases

Vendors provide their own monitoring solutions for GPU metrics.

[tenzen-y]

@eero-t Thank you for creating this!

According to the release note, those metrics can't be supported since K8s v1.25+.

https://github.com/google/cadvisor/releases/tag/v0.47.0

So, we should remove the docs for the next training operator release!

ref: #1803

cc: @kubeflow/wg-training-leads

[tenzen-y]

/help
/good-first-issue

[tenzen-y]

/remove-help
/remove-good-first-issue