[IMP] website, website_slides: adapt get_base_url for website

On `website` itself there is no `website_id` so the domain has to be read from
`self` directly.

Also oversight of forward-port 039e6bae:
On `slide.channel` the override that was done in an earlier version is doing the
same as the generic override that was introduced later, so it can be removed.

closes odoo/odoo#35839

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