To delete, click on the Control Column (displayed as three little dots) next to the desired line item and select Delete.
Generally, anytime a user deletes an item, a popup message will appear asking for confirmation from the user.
Alternatively, you can delete a Task from the View Task section - opened in the right-panel when you click on a task - by clicking on Delete Task.
Note that deleting Tasks is rarely the right call, and you should consider marking Tasks ‘Completed’ instead. This gives you a record of when they were completed as well as an optional Completion Summary, and lists them in the Completed section of the firm tasks as opposed to nowhere.