[FIX] sale: exclude all attributes

With the demo data:
- Go to Sale > Products > Products, search for 'Customizable Desk'
- Click on 'Configure Variants'
- For variant 'Legs: Steel', add all attribute values for the
  'Conference Chair'
- Create a SO, add a line with 'Configure Product'
- Select the 'Customizable Desk' with 'Legs: Steel', add

Boom boom, traceback at template rendering at
`first_possible_combination[variant_id_index] == value_id`

This is because the `parent_combination` is not taken into account when
evaluating `_is_add_to_cart_possible`.

closes odoo/odoo#31496

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>