Remove obsolete Msf::PayloadSet#add_module

MSP-9228
Remove obsolete Msf::Payload#add_single

MSP-9228
Remove obsolete Msf::PayloadSet#add_stage

MSP-9228
Remove obsolete Msf::PayloadSet#build_payload

MSP-9228
Remove obsolete Msf::PayloadSet#instance

MSP-9228
Remove Msf::PayloadSet#stagers and #_stagers

MSP-9228
Remove obsolete Msf::PayloadSet#stages and #_stages

MSP-9228
Remove #on_module_reload

MSP-9228

Blob-cache was replaced by Metasm cache that is keyed directly on the
asm string, so no need to explicitly clear the cache since the key will
properly update.
Despecialize Msf::PayloadSet

MSP-9228

After removing enough obsolete methods, it became obvious that
Msf::PayloadSet was no longer distinct compared to Msf::ModuleSet, so
remove Msf::PayloadSet and use Msf::ModuleSet for payloads.