This functionality is new as of version 4.0.
While the Purchase Order is in the 'New' Status, Purchase Order Items can be deleted.
If the Purchase Order is not in the 'New' Status, Purchase Order Items that were incorrectly entered, or otherwise not received, can be Cancelled.
Refer to Cancel Ordered Quantity.
To Delete a Purchase Order Item:
On the 'Purchase Order Details' screen, for a 'New' Purchase Order.
Either right-click the item row; OR, click the Hamburger button at the right end of the item row.
Select the 'Delete Item' menu option.
Confirm the action.