Remove use of Util::Cacher from FileServing::Mount::File

Allowing this value to be expirable is superfluous; it is only used on the
master, which never expires its cache. Additionally, it was providing partial
support for an event we don't fully support already (hostname and domain
changing during the lifetime of a master).

Reviewed-By: Jacob Helwig <jacob@puppetlabs.com>
(cherry picked from commit 6a1b65760a0d8c6299d5c6d260dc37b5e0637706)