Remove Puppet::Network::HttpPool keep_alive handling

Keep alive has been disabled since 2008, and seems to have caused problems when
it was enabled before then. Since there doesn't seem to be any push to get it
working again, just remove it to simplify this code.

This also allows us to entirely remove the usage of Puppet::Util::Cacher from
HttpPool.

Paired-With: Jacob Helwig <jacob@puppetlabs.com>