(PUP-3050) Update cfacter option for recent cfacter gem changes.

Now that cfacter implements the Ruby Facter interoperability layer, there
is no need for Puppet to "switch out" the Facter module for the CFacter
module.  libfacter itself will now expose a compatible Facter module in
C++.

Therefore, we no longer need facts.rb or the corresponding spec.
Instead, simply require cfacter and call CFacter.initialize.  This will
replace Ruby Facter with the libfacter implementation.
This only confirms that Puppet calls `CFacter.initialize` when the setting is enabled.  It doesn't verify that CFacter actually initialized since currently cfacter isn't a dependency of Puppet's (i.e. the gem isn't in the Gemfile).  cfacter's gem spec does test the behavior of `initialize` though by calling it and interacting with the Facter module.