To remove a collection of artworks that you've created you'll need to erase the collection label from every Artwork where you applied it.
The collection label will automatically be erased after that.
STEP BY STEP
1) Go to the Artworks section in your dashboard.
2) Go to every artwork in the collection and click on the edit option.
3) Once the artwork form opens up, in the collections section, remove the tag corresponding to the collection you want to remove.
Once no artworks contains the tag corresponding to the collection you want to erase, then the collection will be erased.