tools: remove disabling of already-disabled rule

`require-buffer` is only enabled in the `lib` directory. There is no
need to disable it in `test`.

PR-URL: https://github.com/nodejs/node/pull/6013


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>