[FIX] website: simplify filter_duplicate
[FIX] website: simplify filter_duplicate

The `filter_duplicate()` method was introduced with b0b6b9afb53 for
website_version.
The module was removed on Odoo 12.0 with https://github.com/odoo/enterprise/commit/51c1bb3cca9ae02fc55cb49ba98cec2901a8e273

`filter_duplicate()` can now fit the multi-website behavior more simply.

Closes #29347
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo