Removed inactive Athens, Greece MUG from communities.

I have terminated the MUG in Athens.
Please merge when possible.

This closes #201

site/source/community/user-groups.html.md

@@ -23,9 +23,6 @@ Mesos User Groups (MUGs) are responsible for organizing local meetups, hackathon
 * [Cologne](http://www.meetup.com/Mesos-User-Group-Cologne/)
 * [Hamburg](http://www.meetup.com/Hamburg-Mesos-User-Group/)
 
-### Greece
-* [Athens](http://www.meetup.com/Hellenic-Mesos-User-Group/)
-
 ### India
 * [Bangalore](http://www.meetup.com/Bangalore-Mesos-User-Group/)
 * [Pune](http://www.meetup.com/Pune-Mesos-Users-Group/)