Clean up QGraphicsItemCache::purge()

The old code assigned an empty QPoint to elements contained
by value in a QHash that was cleared out in the very next
step.

That makes no sense, because the operations in the loop cannot
possibly cause a re-entrancy into QGraphicsItemCache, which
would be the only explanation for modifying the state of a
death-row object.

While at it, replace the use of the highly inefficient
(and no longer needed) QMutableHashIterator with C++11
range-for, taking care to iterate over a const reference to
avoid detaches.

Change-Id: Ie3eba0f954644a27932666bc9e97f1ca8f36a578
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/widgets/graphicsview/qgraphicsitem.cpp

@@ -1373,12 +1373,9 @@ void QGraphicsItemCache::purge()
 {
     QPixmapCache::remove(key);
     key = QPixmapCache::Key();
-    QMutableHashIterator<QPaintDevice *, DeviceData> it(deviceData);
-    while (it.hasNext()) {
-        DeviceData &data = it.next().value();
+    const auto &constDeviceData = deviceData; // avoid detach
+    for (const auto &data : constDeviceData)
         QPixmapCache::remove(data.key);
-        data.cacheIndent = QPoint();
-    }
     deviceData.clear();
     allExposed = true;
     exposed.clear();