Sorry for the late reply. (I do not have much time for OSM atm.)
There is no strict procedure to follow in order to delete a route relation. To satisfy other mappers you might want to take a look at the history and ask the mappers who have added and updated the relation in the past but only the mappers who actively worked on the relation and not the ones who have just split a way or changed a single dual carriage way into two oneway parts.