(FACT-94) Unvendor CFPropertyList

Vendoring CFPropertyList inside of Facter has proven to be very
problematic. Since it's possible to treat CFPropertyList as a real
dependency via gems and can be bundled at build time for DMGs, there's
no real need to vendor it inside of Facter. This commit removes the
vendored code and prevents facter/util/macosx from being loaded on
non-osx platforms.