fs: removing unnecessary nullCheckCallNT

`nullCheckCallNT()` function is not necessary, as we can directly pass
`callback` and `er` to `process.nextTick()`.

PR-URL: https://github.com/nodejs/io.js/pull/1870


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>