eks-prow-build-cluster: Remove the KubeCost deployment

[xmudrii]

We didn't manage to utilize KubeCost to gather data about ProwJobs because ProwJobs are non-managed Pods (e.g. not part of a ReplicaSet/Deployment) and this is not supported well by KubeCost. For that reason, gathering data required a lot of effort and we got similar data to what we have in our dashboards.

@koksay proposed that we remove the KubeCost deployment from the EKS Prow build cluster, so I'm created a ticket to track that.

/priority important-longterm /area infra/aws /area infra /sig k8s-infra