Remove deprecated GetSources() implementations

This is step #5 of 5 of incrementally landing the changes from:
https://webrtc-review.git.corp.google.com/c/src/+/494041/ (a CL that's
pretty big as is).

The overall goal of these CLs is to move the SourceTracker from
ChannelReceive (audio) and VideoReceiveStream2 (video) into
RtpReceiverBase and be accessible on the signaling thread rather than
interfere with the worker (usually joined with network) thread. This
offloads the worker/network thread for managing the SourceTracker and
also moves the tracker closer to the API layer which it implements
(getContributingSources).

In this CL: Remove deprecated GetSources() implementations from media
and call interfaces now that PC layer no longer calls them.

Bug: chromium:463591201, webrtc:42222117
Change-Id: Iee48c4ad97edf19a49ea7def40c2142c2590c218
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/497820
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48460}