(PUP-3275) Remove deprecated `from_pson` method

This commit removes the misnamed and deprecated `from_pson` method.
It is replaced by the `from_data_hash` method.

This commit is part of the Puppet 4 code removal effort.
Merge pull request #3277 from whopper/pup-3275/removal/from_pson

(PUP-3275) Remove deprecated `from_pson` method