product_get_cost_field: standard_price replaced in tree view, provoking incompatibilities

[Guewen Baconnier @ Camptocamp]

In product_get_cost_field, the field standard_price of the product's tree view is replaced by the cost_price field.

Now, if another module wants to use the standard_price as reference, or for the searches, the field is missing.

Instead of being replaced, the field should be set as invisible.

Example of view that is broken when this module is installed (in sale_markup)
    <record model="ir.ui.view" id="sale_markup_product_tree">
      <field name="name">product.markup.view.tree</field>
      <field name="model">product.product</field>
      <field name="inherit_id" ref="product.product_product_tree_view" />
      <field name="arch" type="xml">
        <field name="standard_price" position="after">
          <field name="markup_rate"
                 groups="base.group_sale_manager"/>
        </field>
      </field>
    </record>