Revert the shared-elfldr integration for the umtx2 chain (FW 1.00-5.50):
load_local_elf now fetches payloads/elfldr-ps5.elf from the umtx2 submodule
(stock behavior) instead of ../shared/elfldr-ps5.elf (itsPLK build shared
with slopkit).

- tools/apply_umtx2_patch.sh: prune bundled payloads down to elfldr-ps5.elf;
  sanity check requires payloads/elfldr-ps5.elf as the only payload and no
  ../shared/ reference
- tools/gen_file_registry.py / tools/build_host.py: stop excluding
  umtx2/payloads/ so the elfldr is embedded and served
- ARCHITECTURE.md / scripts/download_deps.sh: document that umtx2 boots its
  own elfldr; the shared elfldr remains for slopkit only