Clean up dead tunnel code

Running go vet on tunnel package returns:
$ go vet ./...
./default.go:929:2: unreachable code
./link.go:104:2: unreachable code
./listener.go:184:2: unreachable code
./session.go:241:2: unreachable code

tunnel/session.go

@@ -237,8 +237,6 @@ func (s *session) Send(m *transport.Message) error {
 	case <-s.closed:
 		return io.EOF
 	}
-
-	return nil
 }
 
 // Recv is used to receive a message