[FIX] point_of_sale: rounding issue with cash rounding

The method that computs what is due and the change of an order is based
on the selected payment line which is wrong, because if you add and
remove a payment line, none will be seleced, that lead to issue in this
case because a change is computed, and it won't match the invoices
because an amount_return is sent to server.

To reproduce you can follow this simply procedure:
    - set rounding half-up to 0.05
    - create a product at 0.98
    - open POS and create an order with the product
    - go to payment screen
    - add cash payment (1€ auto filled)
    - add bank payment (It'll autofill -0.02 not really a problem)
    - remove the bank payment => It'll show 0.02 due (it is caused because of no payment method is selected)
    - Set a customer and check the invoice => unbalanced (0.02 probably because it is set in amount_return of the request)

closes odoo/odoo#70825

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>