[FIX] website: restore proper uninstallation of the website app

Before this commit, we had an error following those steps:

1. Install website app.
2. Customize the first website default theme settings (like the
   font-size of the header logo).
3. Uninstall the website app.
4. Reinstall the website app.
5. Try step (2) again => It does not work.

This is due to custom SCSS attachments which were not properly removed
after we uninstalled the app. The problem was actually already solved
at some point with [1] but was later broken by [2] which prevented to
properly call the `unlink` method for the only remaining website.

[1]: https://github.com/odoo/odoo/commit/2f361bec36dff09181b96d140d62c477cdf013a1
[2]: https://github.com/odoo/odoo/commit/93a2e85ae7b11f9a390f40f74e6d78e1ec876fc3

Fixes https://github.com/odoo/odoo/issues/44240


opw-2947919

closes odoo/odoo#97852

Signed-off-by: Romain Derie (rde) <rde@odoo.com>