Don't send all sound off message at end of loop

[SanderVocke]

This is overkill, and also we are only doing it on channel 0:

MidiChannel<TimeType, SizeType>::PROC_send_all_sound_off(unsigned frame) {
    auto &buf = mp_playback_target_buffer.value();
    if (buf.first.frames_left() < 1) {
        throw_error<std::runtime_error>(
            "Attempting to play back out of bounds");
    }
    PROC_send_message_value(*buf.second,
        frame, 3, uint8_t {0xB0, 120, 0})
}

Rather, just send whatever we need to send to deactivate active notes.