tst_QGraphicsWidget: remove unused object from qgraphicswidget()

The QGraphicsScene object wasn't used, so can be removed.

Make it so.

Seems to amend the start of the public history (and if there was a
user that has since been removed, we'll get a compile error, so no
need to data-mine the git history further).

Pick-to: 6.8 6.5 5.15
Change-Id: I9eeacde057ee5681ea86c185b66dff618a6f364e
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 235fe36de7d8789cf6f5a393df01337a59874901)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/widgets/graphicsview/qgraphicswidget/tst_qgraphicswidget.cpp

@@ -289,7 +289,6 @@ void tst_QGraphicsWidget::qgraphicswidget()
     QCOMPARE(widget.call_propertyChange(QString(), QVariant()), QVariant());
     widget.call_sizeHint(Qt::PreferredSize, QSizeF());
 
-    QGraphicsScene scene;
     QGraphicsWidget *parent = new QGraphicsWidget;
     SizeHinter *child = new SizeHinter(parent);