Allow to remove items on a WorkOrder

Also, don't let an order to be finished if it doesn't contain items (this
wasn't a problem before since we couldn't remove items)

(cherry picked from commit bbeac4ceab1ef6fdfda1cd32cf3afa4401980ce4)

Change-Id: I880e8388b7441d50a15592c2d2dee7b2d90e5b02