Remove a misleading comment about self.render() behavior.
The former comment suggested that View.render called with 'row' or 'col' would
set cw_row or cw_col, which is not (anymore ?) accurate.
Related to #3696871.
[migractions] add sanity check in rename_relation_type
If the relation is still present in the fs schema, we probably shouldn't
be renaming it (because it's still in use somewhere, possibly in a
different cube). Closes #3608172