input-device: Downgrade over-zealous warning to note

This warning is really harmless, we just discard the event anyway, let's
demote it to a CLUTTER_NOTE() so event backend implementers can see it.

clutter/clutter-input-device.c

@@ -515,7 +515,7 @@ _clutter_input_device_update (ClutterInputDevice *device)
   stage = device->stage;
   if (G_UNLIKELY (stage == NULL))
     {
-      g_warning ("No stage defined for device '%s'",
+      CLUTTER_NOTE (EVENT, "No stage defined for device '%s'",
                     clutter_input_device_get_device_name (device));
       return NULL;
     }