Removing the `whisper_pcm_to_mel_phase_vocoder*` functions

[iboB]

The whisper_pcm_to_mel_phase_vocoder* functions are not used in any of the examples. It seems that they are only to be used with whisper_full_params::speed_up = true which is forbidden

Are they planned to be used in the future? If not, may I remove them?

I'm working on a set of optimizations for the mel spectrogram calculation and it would make my life much easier if I don't have to support them too (and rely on the fact that some of the arguments which are currently provided to log_mel_spectrogram are known at compile time).

Should whisper_full_params::speed_up also be removed?

[ggerganov]

Yes, we can remove the speed_up functionality - it was in experimental stage since the beginning and it didn't seem to provide any value