Remove yamux support (#1526)

## Issue Addressed

There is currently an issue with yamux when connecting to prysm peers. The source of the issue is currently unknown. 

This PR removes yamux support to force mplex negotation. We can add back yamux support once we have isolated and corrected the issue.

beacon_node/eth2_libp2p/src/service.rs

@@ -85,7 +85,7 @@ impl<TSpec: EthSpec> Service<TSpec> {
         debug!(log, "Attempting to open listening ports"; "address" => format!("{}", config.listen_address), "tcp_port" => config.libp2p_port, "udp_port" => discovery_string);
 
         let mut swarm = {
-            // Set up the transport - tcp/ws with noise and yamux/mplex
+            // Set up the transport - tcp/ws with noise and mplex
             let transport = build_transport(local_keypair.clone())
                 .map_err(|e| format!("Failed to build transport: {:?}", e))?;
             // Lighthouse network behaviour
@@ -297,7 +297,7 @@ impl<TSpec: EthSpec> Service<TSpec> {
 }
 
 /// The implementation supports TCP/IP, WebSockets over TCP/IP, noise as the encryption layer, and
-/// yamux or mplex as the multiplexing layer.
+/// mplex as the multiplexing layer.
 fn build_transport(
     local_private_key: Keypair,
 ) -> Result<Boxed<(PeerId, StreamMuxerBox), Error>, Error> {
@@ -312,10 +312,7 @@ fn build_transport(
     Ok(transport
         .upgrade(core::upgrade::Version::V1)
         .authenticate(generate_noise_config(&local_private_key))
-        .multiplex(core::upgrade::SelectUpgrade::new(
-            libp2p::mplex::MplexConfig::new(),
-            libp2p::yamux::Config::default(),
-        ))
+        .multiplex(libp2p::mplex::MplexConfig::new())
         .map(|(peer, muxer), _| (peer, core::muxing::StreamMuxerBox::new(muxer)))
         .timeout(Duration::from_secs(10))
         .timeout(Duration::from_secs(10))