Remove left/right check on create

It was introduced in changeset c7804c288ca3 but tools like proteus will set the
value on creation because there is a default value on such field as they are
required. So such test is not doable on creation.

issue3019

trytond/model/modelsql.py

@@ -381,11 +381,6 @@ class ModelSQL(ModelStorage):
             if (isinstance(field, fields.Many2One)
                     and field.model_name == cls.__name__
                     and field.left and field.right):
-                if (field.left in modified_fields
-                        or field.right in modified_fields):
-                    raise Exception('ValidateError',
-                            'You can not update fields: "%s", "%s"' %
-                            (field.left, field.right))
                 if len(new_ids) == 1:
                     cls._update_tree(new_ids[0], k, field.left, field.right)
                 else: