Add ability to remove vouchers when editing an order in the control panel

[engram-design]

At the moment, we provide an additional helpers when editing an order in the control panel to apply a voucher to a cart. This is to get around the lack of flexibility with Commerce's Vue order-edit screen.

However, we'll need to add a way to remove voucher codes from an order, as currently you have to start the order again to remove it.

[iamkeir]

+1