[FIX] product: remove duplicated field product.template form view

When the purchase model is installed, there are two checkbox with
label 'Can be Purchased'. This error is due the rev https://github.com/odoo/odoo/commit/e945b86d5a61e53a9104ab6cc4ed4a1c060587e4
where we moved purchase related fields from module product to an
inherited view in purchase module.

To fix this, we removed the 'purchase_ok' field that was left in
the product_template view.

addons/product/views/product_views.xml

@@ -33,10 +33,6 @@
                                 <field name="sale_ok"/>
                                 <label for="sale_ok"/>
                             </div>
-                            <div>
-                                <field name="purchase_ok"/>
-                                <label for="purchase_ok"/>
-                            </div>
                         </div>
                     </div>
                     <notebook>