remove the inbuilt synth drum kit (superseded by sampled drum_kit) nf-xbac ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/209 by tools/beads/import_github.py's reconcile pass.

From the #188 review: 'synth drums can be fully removed at this point imo.' Bigger than a follow-up commit: DrumKitSource spans models, Voice/PlaybackDriver synthesis, PianoRoll drum lanes, SoundEditor testers, BaseSounds, and 4 test files — plus save migration (existing saves/library entries with kind: drum-kit sounds must load; plan: keep the type deserializable as a legacy stub and convert to SampleSource(drum_kit) on load). Tracking here so it lands as one reviewable deletion PR.