[FIX] website_sale: avoid adding website on customer when not needed

Before this commit, when an address would be edited or created from checkout,
we would always add a website_id on it.
(values_postprocess -> _checkout_form_save)

That would be incorrect if the website has not 'specific user account' enabled.

It would lead to an error as the user would not be able to connect to another
website. Indeed, the billing address is stored on the partner directly. Since
the res.user website_id is a related to the partner one, the user would then be
restricted to that website.

Fixes https://github.com/odoo/odoo/issues/34308



closes odoo/odoo#34396

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>