doc: remove mailing list

We removed the mailing list from the README and other places quite some
time ago. Core devs don't monitor it much. However, it is still linked
in a couple places in the docs directory. Remove those links.

PR-URL: https://github.com/nodejs/node/pull/23932
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/index.md

@@ -61,4 +61,3 @@
 <div class="line"></div>
 
 * [GitHub Repo & Issue Tracker](https://github.com/nodejs/node)
-* [Mailing List](https://groups.google.com/forum/#!forum/nodejs)

doc/node.1

@@ -389,10 +389,6 @@ GitHub repository & Issue Tracker:
 .Sy https://github.com/nodejs/node
 .
 .Pp
-Mailing list:
-.Sy http://groups.google.com/group/nodejs
-.
-.Pp
 IRC (general questions):
 .Sy "chat.freenode.net #node.js"
 (unofficial)