[FIX] website_sale_comparison: archived (or deleted) products

Archived or deleted products shouldn't be counted in the comparison
widget count anymore.

The current behavior did only reset the cookie (and the count) when
products were added/removed from the comparison, but didn't consider
the case when the products currently in the cookie weren't all existing/valid.

Fixes #44049

closes odoo/odoo#44309

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