Follow-up from "Add Sample Data" - Move clear_consumed_relations logic to main consume_relation method

The following discussion from !41699 (merged) should be addressed:

• @ayufan started a discussion: (+4 comments)

  This feels like a hacky solution that should rather be implemented as an argument of consume_relation:

  def consume_relation(importable_path, key, mark_as_consumed: true)

  And likely rewrite this method a little:

  def dates
     return unless relation_reader.legacy?
  
     DATE_MODELS.flatten_map do |model|
       relation_reader.consume_relation(@importable_path, model, mark_as_consumed: false).map do |model| 
         model.first['due_date']
       end
     end
  end

  This would already return a flattened map of dates.