remove references to techpreview, extras, and soa-tooling sites from JBDS 6.0 and 6.0-staging sites' index.html pages

Remove references to techpreview, extras, and soa-tooling sites from JBDS 6.0 and 6.0-staging sites' index.html pages:

https://devstudio.jboss.com/updates/6.0-staging/
https://devstudio.jboss.com/updates/6.0/

Max also requests:

(2012-11-26 19:08:48) manderse@redhat.com: Add a screenshot of Central pointing to the extensions page would be more relevant