message: clippy: fix let_unit_value warnings
diff: @@ -76,10 +76,9 @@ pub fn save_metas(segment_metas: Vec<SegmentMeta>,
  };
  let mut w = try!(serde_json::to_vec_pretty(&metas));
  try!(write!(&mut w, "\n"));
  - let res = directory.atomic_write(&META_FILEPATH, &w[..])?;
  + directory.atomic_write(&META_FILEPATH, &w[..])?;
  debug!("Saved metas {:?}", serde_json::to_string_pretty(&metas));
  - Ok(res)
  -
  + Ok(())
  }
  