don't rely on webmidi (#1350)

* don't rely on webmidi

* updated pxt ref

sim/state/midi.ts

@@ -1,7 +1,6 @@
 namespace pxsim.control {
     export function __midiSend(data: RefBuffer) {
         const b = board();
-        pxsim.AudioContextManager.sendMidiMessageAsync(data)
-            .done();
+        pxsim.AudioContextManager.sendMidiMessage(data);
     }
 }