kube-prometheus: Fix grafana label

jsonnet/kube-prometheus/kube-prometheus.libsonnet

@@ -15,13 +15,7 @@ local configMapList = k.core.v1.configMapList;
     namespace: k.core.v1.namespace.new($._config.namespace),
   },
   grafana+:: {
-    local serviceLabels = {
-      app: 'grafana',
-    },
     dashboardDefinitions: configMapList.new(super.dashboardDefinitions),
-    service+: {
-      labels+: serviceLabels,
-    },
     serviceMonitor: {
       apiVersion: 'monitoring.coreos.com/v1',
       kind: 'ServiceMonitor',
@@ -31,7 +25,9 @@ local configMapList = k.core.v1.configMapList;
       },
       spec: {
         selector: {
-          matchLabels: serviceLabels,
+          matchLabels: {
+            app: 'grafana',
+          },
         },
         endpoints: [
           {