Revert "Restore a temporary QQmlInstanceModel::object() overload"

This reverts commit 823428d. No more
needed after qt3d commit 78f77f80057b1e77c3f47d52de3e0b3f0c5d8d6e that
took the new API into use.

Change-Id: I4acb707cf363a625ae8e90c4560a0d5140cc4011
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/qml/types/qqmlobjectmodel_p.h

@@ -75,7 +75,6 @@ class Q_QML_PRIVATE_EXPORT QQmlInstanceModel : public QObject
 
     virtual int count() const = 0;
     virtual bool isValid() const = 0;
-    QObject *object(int index, bool async) { return object(index, async ? QQmlIncubator::Asynchronous : QQmlIncubator::AsynchronousIfNested); }
     virtual QObject *object(int index, QQmlIncubator::IncubationMode incubationMode = QQmlIncubator::AsynchronousIfNested) = 0;
     virtual ReleaseFlags release(QObject *object) = 0;
     virtual void cancel(int) {}