(PUP-3355) Remove PSON document_type

This commit removes the encapsulation in the PSON serialization format.
Puppet only did this when using PSON on certain endpoints, other formats
like YAML or MsgPack never did this.

This is backwards compatible with older Puppet versions as they throw
away the info if it is there. But it is backwards incompatible for other
consumers of the PSON APIs.
(PUP-3355) Remove Puppet::Util::Pson

Remove the now unused Puppet::Util::Pson class.
(PUP-3355) Skip object creation during PSON parsing

The previous commits prevented newer agents from being able to parse
PSON responses from older masters, which use the "short" registered
document type name. For example, the node response message from an older
master contains "document_type":"Node", and the agent would fail to
PSON.parse it, because it no longer registers document types.

This commit removes the logic from the PSON parser for constructing
arbitrary object types, based on document_type or otherwise. This way
the PSON parser just returns safe object types (Hash, Array, or
primitive). If the master's response includes a "data" envelope with
a document_type, it will be returned in the parsed data, and promptly
ignored by the PSON format handler.
Merge branch 'ticket/master/PUP-3355-remove_document_type'

* ticket/master/PUP-3355-remove_document_type:
  (PUP-3355) Skip object creation during PSON parsing
  (PUP-3355) Remove Puppet::Util::Pson
  (PUP-3355) Remove PSON document_type