protocols/rendezvous: Remove unnecessary mapping to StreamMuxerBox (#2668)

Calling `.boxed()` will already map the muxer in a `StreamMuxer` box.
This mapping is therefore unnecessary.

Author: thomaseizinger

protocols/rendezvous/tests/harness.rs

@@ -57,7 +57,6 @@ where
             MplexConfig::new(),
         ))
         .timeout(Duration::from_secs(5))
-        .map(|(peer, muxer), _| (peer, StreamMuxerBox::new(muxer)))
         .boxed();
 
     SwarmBuilder::new(transport, behaviour_fn(peer_id, identity), peer_id)