purchase_usability: fix perf issue: we can't show a non-computed field in tree view

purchase_usability/purchase_view.xml

@@ -41,9 +41,11 @@
     <field name="model">purchase.order</field>
     <field name="inherit_id" ref="purchase.purchase_order_tree"/>
     <field name="arch" type="xml">
+        <!-- We can't show a non-stored computed field in tree view
+             otherwise we have perf issues
         <field name="state" position="after">
             <field name="is_shipped" invisible="not context.get('show_purchase', False)"/>
-        </field>
+        </field>  -->
         <!-- the 'origin' field can be very long ; it can list a lot of MO or OP!
         I think limiting the size of the field would not be the best option,
         because the info it carries can be interesting. So we just remove it from