[FIX] point_of_sale: wrong variable name

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

addons/point_of_sale/static/src/js/models.js

@@ -1450,7 +1450,7 @@ exports.Orderline = Backbone.Model.extend({
             var amount = self._compute_all(tax, base, quantity, false);
 
             if(!tax.price_include || base_gaps.length == 0)
-                return tax_amount;
+                return amount;
 
             var new_gap = base_gaps[base_gaps.length - 1] - amount;