Modify NewVolumeManager() function return value

Since function NewVolumeManager() will always return vm and nil, we do not need the second return value, it will always be nil.

pkg/kubelet/runonce_test.go

@@ -99,7 +99,7 @@ func TestRunOnce(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to initialize VolumePluginMgr: %v", err)
 	}
-	kb.volumeManager, err = volumemanager.NewVolumeManager(
+	kb.volumeManager = volumemanager.NewVolumeManager(
 		true,
 		kb.nodeName,
 		kb.podManager,