Decommission unused ES cluster

While we work on defining a plan for https://gitlab.com/gitlab-com/infrastructure/issues/1597 we can get rid of the 3-node ES cluster that we're currently not using.

• Clean-up resources in Azure as defined in https://gitlab.com/gitlab-com/runbooks/blob/master/howto/remove-node.md
• Remove Prometheus scrape targets for gitlab-elasticsearch