Cleanups of various unused methods in RTCPeerConnection and friends

Change-Id: I53ed22648beda8c499354316abfdf6dcdd233d8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5428786


Auto-Submit: Tony Herre <toprice@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283131}