[FIX] account_factux: discount taken only for gross price

How to reproduce the bug:

- Install the account app
- Get a FacturX invoice with a discount on item
- Upload the invoice
- Check that the price of the discounted item takes the
  discount into account

Bug:

If you want to upload a FacturX invoice that contains a discount,
Odoo will take the original price into account instead of the
discounted one.

opw-2705291

closes odoo/odoo#84228

Signed-off-by: Adrien Minet <admi@odoo.com>
Signed-off-by: William André (wan) <wan@odoo.com>