transport: remove unused bufWriter.onFlush() (#5464)

Author: mitchsw

internal/transport/http_util.go

@@ -322,8 +322,6 @@ type bufWriter struct {
 	batchSize int
 	conn      net.Conn
 	err       error
-
-	onFlush func()
 }
 
 func newBufWriter(conn net.Conn, batchSize int) *bufWriter {
@@ -360,9 +358,6 @@ func (w *bufWriter) Flush() error {
 	if w.offset == 0 {
 		return nil
 	}
-	if w.onFlush != nil {
-		w.onFlush()
-	}
 	_, w.err = w.conn.Write(w.buf[:w.offset])
 	w.offset = 0
 	return w.err