[FIX] point_of_sale: correctly add Global Discount with fiscal position

Previously, it was not possible to apply the Global Discount in cases
where there was a fiscal position with tax included in the price. This
was because of using in the `get_taxes` function, which do not return
taxes after fiscal position.

opw-3267243

closes odoo/odoo#118837

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>