(PUP-3308) Remove deprecation warning about cron purge change

This commit removes deprecation warnings for the :purge attribute
of the cron resource. Previously, when 'purge' was set to true
for unmanaged cron resources, Puppet would fire off a warning
about the 'purge' option becoming more aggressive in future versions.

This update is part of the code removal effort for Puppet 4.
Merge pull request #3147 from whopper/pup-3308/remove-cron-purge-warning

(PUP-3308) Remove deprecation warning about cron purge change