doc: remove bad link to irc info

Remove a link that no longer appears to point to information about
Node.js IRC channels.

PR-URL: https://github.com/nodejs/node/pull/24967


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>