Remove use of Util::Cacher in FileServing::Configuration

This class was using Util::Cacher for its singleton instance, when that was
unnecessary. The FileServing::Configuration instance already manages whether or
not to reparse its config file, based on whether it has changed. Thus, there is
no need for it to be manually expired via the cacher.

Reviewed-By: Jacob Helwig <jacob@puppetlabs.com>