fs: remove unused branches

In a few places the code was refactored to use `maybeCallback` which
always returns a function. Checking for `if (callback)` always returns
true anyway.

PR-URL: https://github.com/nodejs/node/pull/4795


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: thefourtheye <thechargingvolcano@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>