(maint) Remove unused property 'munger' attribute

The munger attribute was added to Puppet::Property in 1d739731, but the
code that would have used it was always commented out and later removed
entirely. Since this attribute isn't respected we can remove it.
Merge pull request #2923 from adrienthebo/maint/puppet-4/remove-unused-munger

(maint) Remove unused property 'munger' attribute