(maint) Remove unused property 'validater' attribute

The validater 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 #2924 from adrienthebo/maint/puppet-4/remove-unused-validater

(maint) Remove unused property 'validater' attribute