Don't depend on futures-io by default

quinn/Cargo.toml

@@ -14,7 +14,7 @@ edition = "2018"
 all-features = true
 
 [features]
-default = ["native-certs", "tls-rustls", "ring", "futures-io"]
+default = ["native-certs", "tls-rustls", "ring"]
 # Records how long locks are held, and warns if they are held >= 1ms
 lock_tracking = []
 # Provides `ClientConfig::with_native_roots()` convenience method