TreeModel: Make debug help code less intrusive

Has not been needed for a while.

Change-Id: I141725d77343c4afa9907fde6af8e283f2744c88
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/libs/utils/treemodel.h

@@ -204,8 +204,6 @@ private:
     QModelIndex indexFromItemHelper(const TreeItem *needle,
         TreeItem *parentItem, const QModelIndex &parentIndex) const;
 
-    void checkIndex(const QModelIndex &index) const;
-
     TreeItem *m_root; // Owned.
 };