Remove unused variable in qwindowsmousehandler.cpp.

Change-Id: Iacfeec396c2eeef973b62bf78c698aa206b01c41
Reviewed-by: Andy Shaw <andy.shaw@digia.com>

src/plugins/platforms/windows/qwindowsmousehandler.cpp

@@ -354,7 +354,6 @@ static bool isValidWheelReceiver(QWindow *candidate)
 bool QWindowsMouseHandler::translateMouseWheelEvent(QWindow *window, HWND,
                                                     MSG msg, LRESULT *)
 {
-    const Qt::MouseButtons buttons = keyStateToMouseButtons((int)msg.wParam);
     const Qt::KeyboardModifiers mods = keyStateToModifiers((int)msg.wParam);
 
     int delta;