[FIX] point_of_sale: make sure FP is not applied twice on refund

current behavior:
When a product with tax included is sold with a fiscal position that
match the tax to a tax of 0%, then when you refund this order the
fiscal is applied a second time. This result in the 15% tax removed 2
times and the price of the product is incorrect.

steps to reproduce:
- Create a product with 15% tax included
- Create a fiscal position that match the tax to a tax of 0%
- Create a POS with this fiscal position
- Open PoS, and make an order with the product
- Applyy the fiscal position
- Refund the order
- The price of the product is not the same as the one of the original
  order.

opw-3371028

closes odoo/odoo#133620

Signed-off-by: Robin Heinz (rhe) <rhe@odoo.com>