message: blobcache: remove useless rate limiter code
  In fact the limiter code has been moved to another thread in which
  merged requests are generated. So the removed code is not necessary
diff: @@ -456,19 +456,6 @@ fn kick_prefetch_workers(cache: &Arc<BlobCache>) {
  blobcache.metrics.prefetch_mr_count.inc();
  }
  - // FIXME: Remove this as it is limited when merging requests
  - if let Some(ref limiter) = blobcache.limiter {
  - let cells = NonZeroU32::new(blob_size).unwrap();
  - if let Err(e) = limiter
  - .check_n(cells)
  - .or_else(|_| block_on(limiter.until_n_ready(cells)))
  - {
  - // `InsufficientCapacity` is the only possible error
  - // Have to give up to avoid dead-loop
  - error!("{}: give up rate-limiting", e);
  - }
  - }
  -
  blobcache
  .metrics
  .prefetch_data_amount
  