remove undocumented requirement to include concat::setup in manifest

Unless the class `concat::setup` has been manually included into the
manifest before using the `concat` / `concat::fragment` defined types,
the puppet master will generate this warning while compiling the catalog.

    Tue Oct 15 14:05:06 -0700 2013 Scope(Concat[/etc/exports]) (warning):
    Could not look up qualified variable 'concat::setup::root_group'; class
    concat::setup has not been evaluated

The need to `include concat::setup` directly into the manifest has never
been part of the documented API.