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: #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/_toc.md

@@ -55,4 +55,3 @@
 <div class="line"></div>
 
 * [GitHub Repo & Issue Tracker](https://github.com/nodejs/node)
-* [Mailing List](https://groups.google.com/group/nodejs)

doc/node.1

@@ -366,9 +366,6 @@ Documentation:
 GitHub repository & Issue Tracker:
 .ur https://github.com/nodejs/node
 
-Mailing list:
-.ur http://groups.google.com/group/nodejs
-
 IRC (general questions):
 .ur "chat.freenode.net #node.js"
 (unofficial)