Remove thin support

This implements the change discussed in issue https://github.com/puppetlabs-operations/puppet-puppet/issues/110, removing support for the thin app server. I like thin, but nobody seems to run puppet masters in it and it seems to needlessly add maintenance overhead to puppet-puppet.

If anybody does want to keep it, please comment here or on https://github.com/puppetlabs-operations/puppet-puppet/issues/110.