This is a preparatory step in deleting the ChannelManager class.
Also delete some declarations whose implementation was previously removed.
Bug: webrtc:13931
Change-Id: I8764c00fa696932e79fcfe17550ef2490d6a1ed1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262804
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36923}