buffer: remove unneeded eslint-disable comment

lib/buffer.js uses a function declaration for `Buffer`. So it never
uses an instance of `Buffer` in the global scope. Therefore the
disabling of the `require-buffer` custom rule is not needed. Remove the
comment.

PR-URL: https://github.com/nodejs/node/pull/11906


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