doc: remove references to deleted freenode channels

PR-URL: https://github.com/nodejs/node/pull/39047


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>