Remove unused import except when feature is enabled that needs it.

examples/client-transports.rs

@@ -1,6 +1,7 @@
 /// Using the `domain::net::client` module for sending a query.
 use std::net::{IpAddr, SocketAddr};
 use std::str::FromStr;
+#[cfg(feature = "unstable-validator")]
 use std::sync::Arc;
 use std::time::Duration;
 use std::vec::Vec;