(maint) remove dead Puppet.genmanifest code

Commit 422dea05e7 (in 2008) moved the --genmanifest functionality from Puppet to
Puppet::Settings; the Puppet.genmanifest method is no longer used by
anything in Puppet and can be removed.
Merge pull request #2732 from adrienthebo/maint/puppet-4/dead-genmanifest-code

Maint/puppet 4/dead genmanifest code