Revert "fs: make callback mandatory to all async functions"

This reverts commit 9359de9d.

Original Commit Message:

    The "fs" module has two functions called `maybeCallback` and
    `makeCallback`, as of now.

    The `maybeCallback` creates a default function to report errors, if the
    parameter passed is not a function object. Basically, if the callback
    is omitted in some cases, this function is used to create a default
    callback function.

    The `makeCallback`, OTOH, creates a default function only if the
    parameter passed is `undefined`, and if it is not a function object it
    will throw an `Error`.

    This patch removes the `maybeCallback` function and makes the callback
    function argument mandatory for all the async functions.

    PR-URL: https://github.com/nodejs/node/pull/7168


Reviewed-By: Trevor Norris <trev.norris@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/7846


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>