(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.