Detach senders from their send stream on rollback

Rolling back a local description deletes the channels that the rolled
back description created, but the senders kept the ssrc that the
description gave them. A sender left in that state refers to a send
stream that no longer exists: applying a later description that disables
a simulcast layer goes through the media channel instead of the sender's
pre-negotiation state, fails with an empty error, and
SetLocalDescription() reports "Failed updating status for simulcast
layers."

Detach the senders as part of the rollback, before the channels are
deleted, so that they return to the un-negotiated state. The encodings
the application configured are restored after the batched teardown tasks
have run, since detaching a sender clears them.

Fixed: chromium:562462476
Change-Id: I548c5444255f5fcb6e7d2a3519979f6b8510b822
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/503880
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48660}