Removes all external buildout files, except for the version pins of Plone and Teamraum

The idea is to have as little external dependencies as possible. Each dependency can influence the buildout on another machine which I see as somewhat dangerous. I'd prefer this to be self contained.

For now the Teamraum and Plone known good sets are used in addition to the onegov kgs (in this repository). For now this seems sane, but since the Teamraum KGS contains packages which are not public we'll have to see how this goes.