Clean up space used by moveChunk in file system
As per the MongoDB official documentation:

Is it safe to remove old files in the moveChunk directory?


  Yes. mongod creates these files as backups during normal shard
  balancing operations. If some error occurs during a migration, these
  files may be helpful in recovering documents affected during the
  migration.
  
  Once the migration has completed successfully and there is no need to
  recover documents from these files, you may safely delete these files.
  Or, if you have an existing backup of the database that you can use
  for recovery, you may also delete these files after migration.
  
  To determine if all migrations are complete, run
  sh.isBalancerRunning() while connected to a mongos instance.