Delete vegas-leads list

The vegas-leads list: https://lists.torproject.org/cgi-bin/mailman/listinfo/vegas-leads has served us well for some years now: https://lists.torproject.org/pipermail/tor-project/2016-March/000196.html

But with the new plans (more structured interactions between teams), we no longer need a separate list for coordination between teams. So it is time to retire (delete) the list.

It has no archives, so it should be easy to remove.

Thanks!