remove code not tested thoroughly

docs/modules/audio/cepstrogram.rst

docs/modules/audio/hpss.rst

docs/modules/utils.rst

docs/modules/utils/stats.rst

madmom/audio/cepstrogram.py

madmom/audio/chroma.py

madmom/audio/filters.py

madmom/audio/hpss.py

madmom/audio/spectrogram.py

madmom/utils/__init__.py

madmom/utils/stats.py

setup.py

tests/test_audio_filters.py

tests/test_audio_spectrogram.py