By "Library" you mean "collection"? A group _is_ a library, so deleting a library in a group isn't possible. To delete a collection, right-click the collection and select "delete collection" or "delete collection and item" -- you'll want to do this in the local app, not in the online library (I think you _can_ do this online, but more of a hassle)
To delete a collection, right-click the collection and select "delete collection" or "delete collection and item" -- you'll want to do this in the local app, not in the online library (I think you _can_ do this online, but more of a hassle)