hashtable: delete assignment operator instead of poisoning it

Use the C++11 approach.

hashtable.hxx.in

@@ -73,7 +73,7 @@ struct _Hashtable_node
   void public_method_to_quiet_warning_about_all_methods_private();
 
 private:
-  void operator=(_Hashtable_node<_Val> const&); // poison node assignment
+  void operator=(_Hashtable_node<_Val> const&) = delete;
 };
 
 template <class _Val, class _Key, class _HashFcn, class _ExtractKey,