When you no longer need a Card Collection you can easily delete it. This will delete the collection (container) and move all of its contents (cards) to your Default Collection.
Open the Card collection and select the red X in the top right corner of the collection window:
Select "Yes Delete Collection." This will remove the collection but keep all the cards that were in the collection. These cards will be moved to your Default Collection where you can Delete them or Move them to other collections.