refactor: remove unused isConnectionReset function

Author: yuuki

server.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"log/slog"
 	"net"
-	"strings"
 	"sync"
 	"time"
 
@@ -176,17 +175,6 @@ func handleConnection(conn net.Conn) error {
 	}
 }
 
-func isConnectionReset(err error) bool {
-	if err == nil {
-		return false
-	}
-	// Check for common connection reset patterns
-	errStr := err.Error()
-	return strings.Contains(errStr, "connection reset by peer") ||
-		strings.Contains(errStr, "broken pipe") ||
-		strings.Contains(errStr, "write: broken pipe")
-}
-
 func (s *Server) serveUDP(ctx context.Context) error {
 	lc := net.ListenConfig{}