Revert "Provide thread-safe channel access during transport changes"

This reverts commit f37609610b40f115922b552fc521a5e75f735ace.

Reason for revert: We need more awareness of the channels on the network
thread to cover all the cases when the transport changes. This CL didn't
catch all of them so we'll need to revisit.

Original change's description:
> Provide thread-safe channel access during transport changes
>
> Introduce a mechanism to safely access active channels on the network
> thread during transport-related operations. Previously,
> OnTransportChanged traversed transceivers to find the associated
> channel, which risked accessing signaling-thread-guarded objects from
> the network thread (both the channel() accessor and list of
> transceivers).
>
> This change adds UpdateNegotiatedChannels to provide a snapshot of
> active channels to the network thread before performing operations such
> as SetRemoteDescription or RollbackTransports. The lookup map,
> negotiated_channels_, is guarded by the network thread and populated
> temporarily during these calls.
>
> Abseil's cleanup utility ensures the map is cleared after the transport
> operation completes, maintaining proper state lifecycle.
>
> Bug: webrtc:470709349, webrtc:42222849
> Change-Id: I7d1ab7c4f5d8cc81feb91c9b4ebeeaf993a6107d
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/435761
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46535}

Bug: webrtc:470709349, webrtc:42222849, chromium:472458706, chromium:472031892
Change-Id: Id605ae79bcec868fccb4a11c2b77f854f937b535
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/440100
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46562}