QmlDesigner: Adapt to C++ 17 [[maybe_unused]]

It has the advantage to to move the attribute to the declaration instead
of using a workaround.

Change-Id: I08b712f146a0625d0367657c31d6c1e5f7caec41
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/edit3d/edit3dcanvas.cpp

@@ -142,10 +142,8 @@ void Edit3DCanvas::keyReleaseEvent(QKeyEvent *e)
     QWidget::keyReleaseEvent(e);
 }
 
-void Edit3DCanvas::paintEvent(QPaintEvent *e)
+void Edit3DCanvas::paintEvent([[maybe_unused]] QPaintEvent *e)
 {
-    Q_UNUSED(e)
-
     QWidget::paintEvent(e);
 
     QPainter painter(this);