doc: remove duplicate content from readline doc

Remove a duplicate sentence from the `rl.close()` method description.

PR-URL: https://github.com/nodejs/node/pull/8497


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>