helm deleteコマンドだけだと、kunernetesのCRD(Custom Resource Definitions)は削除されません。
CRDはkubectl delete crdコマンドで個別に削除をします。
$ helm delete --purge <release_name>
$ kubectl delete crd prometheuses.monitoring.coreos.com
$ kubectl delete crd prometheusrules.monitoring.coreos.com
$ kubectl delete crd servicemonitors.monitoring.coreos.com
$ kubectl delete crd alertmanagers.monitoring.coreos.com