doc: remove Roadmap Working Group

Refs: https://github.com/nodejs/CTC/issues/16
PR-URL: https://github.com/nodejs/node/pull/9545


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Roman Reiss <me@silverwind.io>