Delete deprecated mailing list from README

http://list.mig.ninja/listinfo/list
Service Unavailable

README.md

@@ -123,8 +123,6 @@ Discussion
 Join **#mig** on [irc.mozilla.org](https://wiki.mozilla.org/IRC) (use a web
 client such as [mibbit](https://chat.mibbit.com)).
 
-We also have a [public mailing list at list@mig.ninja](http://list.mig.ninja/listinfo/list).
-
 Documentation
 -------------
 All documentation is available in the 'doc' directory and on http://mig.mozilla.org .