(PUP-3273) Remove deprecated Puppet::FileCollection

Puppet::FileCollection and Puppet::FileCollection::Lookup were removed
of all functionality about 2 years ago. This commit removes them
as part of the Puppet 4 code removal effort.
Merge pull request #3264 from whopper/pup-3273/removal/file_collection

(PUP-3273) Remove deprecated Puppet::FileCollection