[FIX] website_sale: remove reward line from cart

Steps to reproduce:
- go to e-commerce;
- add a product to the cart;
- add a gift card;
- remove the gift card form the cart.
(the user must not belong to the group 'base.group_system')

Issue:
A user error appears.

Cause:
The gift card is a product that cannot be added
to the cart like the others.
Therefore, an error is triggered.

Solution:
Make the error trigger only when adding products to the cart.

opw-3114090

closes odoo/odoo#109654

X-original-commit: 9cc699b89688261220aeb39a99fd866c27808d0a
Signed-off-by: William Braeckman (wbr) <wbr@odoo.com>
Signed-off-by: Lefebvre Thomas (thle) <thle@odoo.com>

addons/website_sale/models/sale_order.py

@@ -129,7 +129,7 @@ class SaleOrder(models.Model):
             raise UserError(_('It is forbidden to modify a sales order which is not in draft status.'))
 
         product = self.env['product.product'].browse(product_id).exists()
-        if not product or not product._is_add_to_cart_allowed():
+        if add_qty and (not product or not product._is_add_to_cart_allowed()):
             raise UserError(_("The given product does not exist therefore it cannot be added to cart."))
 
         if product.lst_price == 0 and product.website_id.prevent_zero_price_sale: