Fixes regression that stopped MIDI hot-plugging from working since v2.7.4

Apparently, if MIDIRestart() is called when the CoreMIDI interaction thread launches:

 * CoreMIDI constants are correctly set in RELEASE build (if not called, they are all zero), but
 * Hot-plugging stops working in DEBUG build. There are no notifications.

Why?

Temporarily disabled call to get back hot-plugging in DEBUG build. Needs more research.

See #106