Hi Holger,
Unfortunately I don’t think I can remove the protocol from that, as not all (mobile) browsers support protocolless URI-s as well as some cache plugins malfunction without using the correct protocol.
This should be resolved via a custom code attached to the admin_url hook. That hook allows you to change parts of the resulting URI-s, and resolves it with any plugins that may use the admin_url(..) function calls. I believe that is the cleanest possible solution.