Click the selection tool 
in the tool bar at the top of the editor window.
Select the objects you want to delete.
To select one object, click it. To select a subset of the objects, click outside of any object and drag across the objects you wish to select. Alternatively, click each object while holding down the Shift key. To select all objects at once, choose Select All from the Edit menu at the top of the editor window. Handles (small black boxes) will appear to indicate that the objects are selected.
Press the Backspace key or choose Clear from the Edit pop up menu at the top of the editor window.