spicec-x11: Drop annoying useless warning

Every time an events comes past where the Window is None (which happens
about once every 5 minutes or so), this annoying "invalid window" message
gets printed. Remove it!

client/x11/platform.cpp

@@ -271,7 +271,6 @@ void XEventHandler::on_event()
 
         XNextEvent(&_x_display, &event);
         if (event.xany.window == None) {
-            LOG_WARN("invalid window");
             continue;
         }