PM_K-1 0.0.22: cleanup -- delete the dead _step_dur fallback

By 0.0.20 every site that needed step duration had switched to L['durs'][step]
(the precomputed tuple from _rebuild_dur). _step_dur was kept as a "legacy
fallback" with a comment claiming it was still used by the tap-tempo path --
which was wrong; grep showed zero callers. Ten lines of unreachable code.

Now that 0.0.21 is sounding clean on the Pico 2, time to trim the sediment. No
behavior change; .mpy shrinks slightly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pico-cp/app.py

@@ -18,7 +18,7 @@
 
 import board, busio, digitalio, analogio, pwmio, displayio, vectorio, time, json, gc, os, supervisor
 supervisor.runtime.autoreload = False   # we write our own files (log + pushed programs); never self-restart
-APP_VERSION = "0.0.21"                   # firmware version (the A/B updater pushes/compares this)
+APP_VERSION = "0.0.22"                   # firmware version (the A/B updater pushes/compares this)
 try:
     import rtc                           # set from the editor's clock SysEx so the log has real timestamps
 except ImportError:
@@ -962,16 +962,6 @@ class App:
             with open("/settings.json", "w") as f: json.dump(d, f)
         except OSError: self.can_write = False
 
-    def _step_dur(self, L, step):                       # legacy fallback (still used by _start_play tap-tempo path)
-        beat = self._beat_ns
-        if L['poly']:
-            m = self.lanes[0]; master_bar = beat * (m['steps'] // m['sub'])
-            return master_bar // L['steps']
-        sub = L['sub']
-        if L['swing'] and sub % 2 == 0:
-            pair = beat // (sub // 2)
-            return (pair * 2) // 3 if (step % sub) % 2 == 0 else pair // 3
-        return beat // sub
     def _rebuild_dur(self, L):                          # cache the per-step ns durations into L['durs'] (tuple lookup is ~10us)
         beat = self._beat_ns
         sub = max(1, L['sub']); steps = max(1, L['steps'])