contrib/kube-prometheus: remove node role

This commit removes get/list/watch on nodes for the Prometheus-k8s
instance, as Prometheus pods do not need that privilege for anything.

jsonnet/kube-prometheus/prometheus/prometheus.libsonnet

@@ -129,7 +129,6 @@ local k = import 'ksonnet/ksonnet.beta.3/k.libsonnet';
       local coreRule = policyRule.new() +
                        policyRule.withApiGroups(['']) +
                        policyRule.withResources([
-                         'nodes',
                          'services',
                          'endpoints',
                          'pods',