To delete a folder select the action "Delete folder" in its action menu (the three dots on the right edge of the folder entry). After you have answered the confirmation question with "Yes" the folder will be deleted with all of its content.

To delete multiple folders select those folders by setting the checkbox left of the folder name and clicking the "Delete" button at the bottom of the file and folder list of the file area. After confirming the deletion all selected folders and their content will be deleted.