Remove Util::Cacher usage from SSL::CertificateAuthority

Allowing the singleton_instance value to be expirable is unnecessary, because
there will never be a need for a different CA instance in the lifetime of a
master. Additionally, the master never expired its cache anyway. This was only
using the cacher so it could be expired for tests, so it can safely be removed.

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