[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.

addons/product/views/product_attribute_views.xml

@@ -93,7 +93,7 @@
         <field name="model">product.template.attribute.value</field>
         <field name="type">tree</field>
         <field name="arch" type="xml">
-            <tree string="Attributes" create="0" delete="0" editable="bottom">
+            <tree string="Attributes" create="0" delete="0">
                 <field name="attribute_id"/>
                 <field name="name"/>
                 <field name="price_extra"/>