(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.