Remove obsolete YARD docs

MSP-9285
Mdm::Session connects to Mdm::Module::Class

MSP-3090

Add Mdm::Session#exploit_class and Mdm::Session#payload_class to
deprecate Mdm::Session#via_exploit and Mdm::Session#via_payload,
respectively.  Mdm::Module::Class gets #exploit_sessions and
 #payload_sesions for the inverses.