For remote servers, we cannot advertise `List-Unsubscribe=One-Click`,
which is specified in RFC 8058[^1] to mean that the `List-Unsubscribe`
URL supports a POST request with no arguments to unsubscribe. Because
we show an interstitial and confirmation page, as this is not just a
mailing list which is disabled if you click the link, it does not
support the mail system performing the unsubscribe for the user.
Remove the inaccurate header for remote servers.
[^1]: https://datatracker.ietf.org/doc/html/rfc8058