transports/dns/: Don't feature flag std::io import (#3027)

Functions like `parse_dnsaddr_txt` depend on the `std::io` import. Given
that the function is not feature flagged, compilation without features
fails.

transports/dns/src/lib.rs

@@ -65,7 +65,6 @@ use libp2p_core::{
 };
 use parking_lot::Mutex;
 use smallvec::SmallVec;
-#[cfg(any(feature = "async-std", feature = "tokio"))]
 use std::io;
 use std::{
     convert::TryFrom,