(PUP-3299) Remove deprecated Puppet::Util::CommandLine methods

This commit removes the deprecated class and instance methods
`available_subcommands` from the Puppet::Util::CommandLine class.
They are replaced by `Puppet::Application.available_application_names`.

This commit is part of the code removal effort for Puppet 4.
Merge pull request #3157 from whopper/pup-3299/removal/commandline_methods

(PUP-3299) Remove deprecated Puppet::Util::CommandLine methods