destroy() {
    this._output = null;
    this._number = null;
    this._octaveOffset = 0;
    this.removeListener();
  }
Unlinks the MIDI subsystem, removes all listeners attached to the channel and nulls the channel
number. This method is mostly for internal use. It has not been prefixed with an underscore
since it is called by other objects such as the `Output` object.

@private