Remove old chef-managed prometheus servers in gitlab-ci project

As per #5834 (closed) and https://gitlab.slack.com/archives/C02KG745VSS/p1635677235000200

We have two prometheus servers in the gitlab-ci GCP project:

• prometheus-01.us-east1-c.gce.gitlab-runners.gitlab.net
• prometheus-01.us-east1-d.gce.gitlab-runners.gitlab.net

They were used in the past to collect metrics from ephemeral VMs created by our runner managers. However, their configuration is already outdated and they soon will be fully replaced K8S deployed ones: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13886.

As there is no "production" usage of those metrics and the servers are generating problems, let's just remove them.

TODO checklist

• Remove all referenes to them from chef-repo 👉 https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/2461
• Remove the nodes (these were manually created so need to be manually deleted).
• Remove the SSL certificates from automatic reneval (if configured).
• Remove the DNS records 👉 https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/4510