[webrtc-pc] Simulcast makes no sense for audio.

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== Simulcast makes no sense for audio. ==
[setParameters()](http://w3c.github.io/webrtc-pc/#dom-rtcrtpsender-setparameters) (and JSPEP) appear kind-agnostic when it comes to allowing multiple encodings.

We should probably limit multiple encodings to video.

I see three options:
 1. TypeError on `sendEncodings.length > 0` in `addTransceiver` for audio, or
 2. TypeError on `sendEncodings.length > 1` in `addTransceiver` for audio.

The first option seems the simplest. The second may add some convenience but would let `rid` be set.

Also, we might want to ignore `active`, and remove `active` and `priority` from `getParameters()`.


Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1813 using your GitHub account