[FIX] product: make product_template_attribute_value tree not editable

Purpose
=======
The product.template.attribute.values can't be edited in an inline tree view
because we need to be able to define exclusions for them in the form view.

closes odoo/odoo#31601

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>