serve: remove deadcode

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

serve.go

@@ -5,9 +5,7 @@ package jsonrpc2
 
 import (
 	"context"
-	"errors"
 	"fmt"
-	"io"
 	"net"
 	"os"
 	"time"
@@ -129,23 +127,3 @@ func Serve(ctx context.Context, ln net.Listener, server StreamServer, idleTimeou
 		}
 	}
 }
-
-// isClosingError reports whether the error occurs normally during the process of
-// closing a network connection.
-//
-// It uses imperfect heuristics that err on the side of false negatives,
-// and should not be used for anything critical.
-func isClosingError(err error) bool {
-	if errors.Is(err, io.EOF) {
-		return true
-	}
-
-	// Per https://github.com/golang/go/issues/4373, this error string should not
-	// change. This is not ideal, but since the worst that could happen here is
-	// some superfluous logging, it is acceptable.
-	if err.Error() == "use of closed network connection" {
-		return true
-	}
-
-	return false
-}