[FIX] purchase: purchase_ok is already available in the view

`purchase_ok` is already in the view,
This field is already set in the view
`product.product_template_form_view`
and `account.product_template_form_view` inherits from this view.

Setting twice a field in a view prevents to correctly change
the field.

opw-641485

addons/purchase/purchase_view.xml

@@ -725,7 +725,6 @@
             <field name="arch" type="xml">
                 <field name="property_account_expense" position="after">
                     <label string="" colspan="2"/>
-                    <field name="purchase_ok" invisible="1"/>
                     <field name="property_account_creditor_price_difference" domain="[('deprecated','=',False)]" attrs="{'readonly':[('purchase_ok', '=', 0)]}" />
                     <newline/>
                 </field>