To delete a Library Folder, click on the Folder you want to delete.
Hover your mouse over the Folder's title and click the ellipsis icon that appears on the right > Delete.
A window will appear asking you to confirm your decision to delete the Folder and its contents. To delete the Folder and its contents, select the "I wish to proceed" checkbox > Delete.
Note: If you do not wish to delete the contents in the Folder, you'll click the "Cancel" link and move the contents to a new Folder before deleting the Folder.
Folders can also be deleted via the Folder Tree Navigation in the left side panel of the Library by hovering your mouse over the Folder that you'd like to delete and clicking the ellipsis icon > Delete.