Make encoderImplementation/decoderImplementation video-only

[henbos]

There has been not enough interest to implement this for audio for years. Maybe easier to update the spec to match the implementation? (Can always revisit later if there is a need for this)

[fippo]

:+1: - for video this makes sense since there are actually different encoders (software vs hardware) but for audio I can only think of the alternatives "libopus" and "builtin" (for g711 + g722)

[alvestrand]

on a principled basis, it would seem that it should be implemented returning the constant string "default". but reflecting reality is kind of important too.

[fippo]

if you send stats elsewhere (even using techniques like delta compression) you don't want the extra bytes or memory