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/link.go

@@ -98,7 +98,6 @@ func (l *link) Close() error {
 		return nil
 	default:
 		close(l.closed)
-		return nil
 	}
 
 	return nil