Remove references to encoded frame transform API from Use Case 1

[tonyherre]

It comes up pretty frequently in discussions about Use Case 1 that changes to the Encoded Frame Transform API are needed if a web app would like to take frames from there and packetize them - missing metadata which would need to be provided etc - which makes it harder to get to the details of this RTPTransport proposal.

To keep the scope of this explainer to focus on sending app-packetized media on an RTPTransport, I'd suggest we remove the explicit Encoded Transform references.
We can take some of the findings we have so far around what would be needed to be exposed to allow apps to packetize WebRTC encoded frames as issues/explainer on https://github.com/w3c/webrtc-encoded-transform.

WDYT @pthatcher, @Philipel-WebRTC, @aboba, @stefanholmer?

[aboba]

Removing the references to Encoded Transform simplifies the explainer, and makes it easier to discuss proposed changes to Encoded Transform separately. The WebCodecs-only (e.g. AAC) and custom (WASM) codec cases may still depend on other API changes (to avoid having to pretend that WebRTC codecs are being used).