rmdir (remotePath) {
    return this.rmrf(remotePath)
      .then(r => {
        return r
      })
      .catch(err => {
        console.error('rm -rf dir error', err)
        return this.removeDirectoryRecursively(remotePath)
      })
  }
rmdir

@param {String} remotePath
https://github.com/mscdex/ssh2/blob/master/SFTP.md
only support rm -rf
@return {Promise}