v-m/vmagent: Use ephemeral storage

The `vmagent` needs a place to spool data it has not yet sent to
Victoria Metrics, but it doesn't really need to be persistent.  As long
as all of the `vmagent` nodes _and_ all of the `vminsert` nodes do not
go down simultaneously, there shouldn't be any data loss.  If they are
all down at the same time, there's probably something else going on and
lost metrics are the least concerning problem.

victoria-metrics/vmagent.yaml

@@ -136,17 +136,5 @@ spec:
       - name: config
         configMap:
           name: vmagent
-  volumeClaimTemplates:
-  - apiVersion: v1
-    kind: PersistentVolumeClaim
-    metadata:
-      name: tmpdata
-      labels:
-        app.kubernetes.io/name: vmagent
-        app.kubernetes.io/component: vmagent
-    spec:
-      accessModes:
-      - ReadWriteOnce
-      resources:
-        requests:
-          storage: 4G
+      - name: tmpdata
+        emptyDir: {}