QFileSystemModel: remove an unneeded const_cast

QAIM::createIndex() takes a const void* pointer, so we don't need to
const_cast the const away.

Amends the start of the public history.

Picking all the way back, because I'm hunting a memleak here, and
can't say, yet, whether these lines need to be modified or
not. Besides, this change is 100% safe: either it compiles and works
as before or it doesn't compile. There is no silent breakage possible.

Pick-to: 6.9 6.8 6.5 5.15
Change-Id: I2f9284540beed9d1ab027ac1e24f1c527537e122
Reviewed-by: Robert Löhning <robert.loehning@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/itemmodels/qfilesystemmodel.cpp

@@ -233,7 +233,7 @@ QModelIndex QFileSystemModel::index(int row, int column, const QModelIndex &pare
     const QFileSystemModelPrivate::QFileSystemNode *indexNode = parentNode->children.value(childName);
     Q_ASSERT(indexNode);
 
-    return createIndex(row, column, const_cast<QFileSystemModelPrivate::QFileSystemNode*>(indexNode));
+    return createIndex(row, column, indexNode);
 }
 
 /*!