Remove coverage of EMG

[m-miedema]

Given that a separate BEP (042) exists to address EMG-type data, we should confirm whether there's any reason not to remove it from the use cases we envision for our physiological data BEP.

[neuromechanist]

See bids-standard#2108, as a pointer to, IMO, a larger issue. Especially this question:

What is the threshold or recommendation for using the data specific modality/recording versus embedding data under other modalities for example, Eye-tracking and EMG can be embedded under EEG as channels.

Several of the physio bio signals are already accommodated as channels within other BIDS modalities. There should be, hopefully, a clarification and guidance, on when and how use that mechanism, when using _physio under other data types and when having the data under the physio directory.