code: void remove(Block block) {
      synchronized (pendingReplications) {
        PendingBlockInfo found = pendingReplications.get(block);
        if (found != null) {
          if (FSNamesystem.LOG.isDebugEnabled()) {
            FSNamesystem.LOG.debug(
                "Removing pending replication for block" + block);
          }
          found.decrementReplicas();
          if (found.getNumReplicas() <= 0) {
            pendingReplications.remove(block);
          }
        }
      }
    }
docs: One replication request for this block has finished.
  Decrement the number of pending replication requests
  for this block.