if we want to use it as 2nd arg to the property function:
- set_limit method should explicitly return None
- remove useless comments
- remove now useless type:ignore comments

This fix mypy error:
Argument 2 to "property"
 has incompatible type "Callable[[rql.stmts.Union, Any], Optional[rql.stmts.Union]]"
 expected "Optional[Callable[[Any, Any], None]]"