Cleanup RTCEncodedFrame clone() method in favour of structuredClone()

The spec discussion is coming towards just supporting structuredClone()
as the best way of achieving these goals. This is already shipped
in Chrome, so we can cleanup this experimental clone() method.

Bug: webrtc:14709
Change-Id: Ib1ac9dd4ed5b639acf10eead00fe98e14ffaf381
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4859934
Reviewed-by: Palak Agarwal <agpalak@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Auto-Submit: Tony Herre <toprice@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Tony Herre <toprice@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1197746}