TreeModel: takeItem should return the item

The usual Qt API for "takeXyz()" methods is to return the item that was
"taken".

Change-Id: Ie144051801487a301b3f13e2857735b65f58150b
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/debugger/watchhandler.cpp

@@ -1251,8 +1251,7 @@ void WatchHandler::removeItemByIName(const QByteArray &iname)
         theWatcherNames.remove(item->exp);
         saveWatchers();
     }
-    m_model->takeItem(item);
-    delete item;
+    delete m_model->takeItem(item);
     updateWatchersWindow();
 }