[FIX] point_of_sale: wrong variable name
[FIX] point_of_sale: wrong variable name

`tax_amount` is not defined here, making it impossible to remove an
order line...

ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,Elico-Corp/odoo_OCB