(PUP-3370) Remove serialization support from indirector request

Support for serializing/deserializing indirector requests was added per
Redmine issue #7080, but was never fully fleshed out and is a rather
sketchy idea in the first place. This commit removes all serialization
logic from Puppet::Indirector::Request.
Merge remote-tracking branch 'upstream/pr/3230'

* upstream/pr/3230:
  (PUP-3370) Remove serialization support from indirector request