NOTICE: No more backward compatibility for MIDI-A/B, 24kHz vocoder and PE

This is a backup release for our further developments.
We will re-arrange the codebase so that no more support for MIDI-A/B SVS modes and 24kHz vocoder will be provided.
At next version of DiffSinger in this forked repository, inference or training with MIDI-A/B SVS, PitchPredictor, the old 24kHz vocoder and PitchExtractor (which MIDI-B relies on) will result in raising errors.
With these clean-ups, we will be able to focus on MIDI-less acoustic model preparation/preprocessing/training/inference and further development for more functionalities.