remove the http3.DataStreamer (#3435)

http3/server.go

@@ -562,11 +562,7 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 	ctx = context.WithValue(ctx, http.LocalAddrContextKey, conn.LocalAddr())
 	req = req.WithContext(ctx)
 	r := newResponseWriter(str, conn, s.logger)
-	defer func() {
-		if !r.usedDataStream() {
-			r.Flush()
-		}
-	}()
+	defer r.Flush()
 	handler := s.Handler
 	if handler == nil {
 		handler = http.DefaultServeMux
@@ -587,10 +583,6 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 		handler.ServeHTTP(r, req)
 	}()
 
-	if r.usedDataStream() {
-		return requestError{err: errHijacked}
-	}
-
 	if panicked {
 		r.WriteHeader(500)
 	} else {