message: remove redudant comment
diff: @@ -251,8 +251,6 @@ impl Rafs {
  // Device should be ready before any prefetch.
  if self.fs_prefetch {
  - // We have to this unsafe conversion because RafsIoReader as a Box pointer can't
  - // be shared between threads safely, even after cloning.
  let sb = self.sb.clone();
  let device = self.device.clone();
  