fix: remove incorrect flush parameter

[piedar]

I have some code like this

flush (path, fd, reply) {
  debug("flush", { path, fd })

  writer.flush(path)
    .then(() => reply(0))
    .catch(err => {
      debug({ err })
      reply(errorToFuseCode(err))
    })
}

but it produces

UnhandledPromiseRejectionWarning: TypeError: reply is not a function

because an extra datasync parameter gets in the way of the documented reply function.

This change removes the incorrect extra parameter.

[mafintosh]

Thanks