SFrameTransform "role" parameter is unneccesary

[bifurcation]

The SFrameTransform role should determined entirely by whether it is attached to an RTCRtpSender or RTCRtpReceiver. It's nonsensical to have role = encrypt when attached to an RTCRtpReceiver, and vice versa.

[youennf]

See #258 (comment) that explains why role can be useful.

[bifurcation]

Another thing that gets simpler if we split the two types! (cf. #262)