[FIX] product: disable variants duplication.

The product copy was fixed in https://github.com/odoo/odoo/pull/38303

 
but the result is still not intuitive.

You could duplicate one of the multiple variants of a template and see 
another variant form opened than the one you just duplicated.

closes odoo/odoo#38725

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>