temporarily revert change which does not work as expected

--- a/src/Tools/Code/code_ml.ML	Sat Apr 25 13:26:24 2020 +0000
+++ b/src/Tools/Code/code_ml.ML	Sat Apr 25 13:26:25 2020 +0000
@@ -692,7 +692,7 @@
                 )
               ];
           in pair
-           (if Code_Namespace.not_private export
+           (if Code_Namespace.is_public export
               then type_decl_p :: map print_classparam_decl classparams
               else if null classrels andalso null classparams
               then [type_decl_p] (*work around weakness in export calculation*)