API: remove routes without version prefix

[josecelano]

API routes are duplicated with and without v1 prefix:

pub fn add(prefix: &str, router: Router, tracker: Arc<Tracker>) -> Router {
    // Without `v1` prefix.
    // We keep the old API endpoints without `v1` prefix for backward compatibility.
    // todo: remove when the torrust index backend is using the `v1` prefix.
    let router = auth_key::routes::add(prefix, router, tracker.clone());
    let router = stats::routes::add(prefix, router, tracker.clone());
    let router = whitelist::routes::add(prefix, router, tracker.clone());
    let router = torrent::routes::add(prefix, router, tracker.clone());

    // With `v1` prefix
    let v1_prefix = format!("{prefix}/v1");
    let router = auth_key::routes::add(&v1_prefix, router, tracker.clone());
    let router = stats::routes::add(&v1_prefix, router, tracker.clone());
    let router = whitelist::routes::add(&v1_prefix, router, tracker.clone());
    torrent::routes::add(&v1_prefix, router, tracker)
}

That was a temporary solution until we changed the backend. The backend was updated so we can remove the routes without prefixes.