Remove ApiEndpointUpdateHandler

Previously, the `mullvad-api` would tell the `mullvad-daemon` that it
wanted a new API endpoint by calling a certain callback
(`ApiEndpointUpdateCallback`), which would asynchronously resolve a new
API endpoint and tell the daemon to punch an appropriate hole in the
firewall for that particular endpoint before the `mullvad-api` crate
would consume it.

The logic of the callback can be moved inside `AccessModeSelector`,
which simplifies the contract between `mullvad-daemon` and `mullvad-api`
somewhat.

mullvad-setup/src/main.rs

@@ -165,7 +165,6 @@ async fn remove_device() -> Result<(), Error> {
                     ApiConnectionMode::try_from_cache(&cache_path)
                         .await
                         .into_repeat(),
-                    |_| async { true },
                 )
                 .await,
         );