[FIX] pos_coupon: correctly filter out global discount

Current behavior:
When a global discount is applied, it is considered as a regular
orderline when computing the order discounts. Because of this if you
applied a coupon and a global discount, the pos would add a discount of
infinity value.

Steps to reproduce:
- Create a promotion with a discount of 10%
- Create a product with no taxes
- Enable the global discount feature, and make sure the Discount product
  has a tax set on it.
- Create a new order, add the product, apply the promotion, and apply
  the global discount.
- The order will have a discount of infinity value.

opw-3094784

closes odoo/odoo#107833

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>