Remove post-initialize channel count check for FFmpegAudioDecoder

During demuxing ffmpeg will decode some frames to verify the true
channel count. Creation of the decoder does not run through the
same process, so after initialization the two may disagree on the
actual stream channel count.

We've previous disabled this for security reasons, but none of the
old test cases fail anymore. We still ensure the AVFrame and the
AVCodecContext match by the time of GetAudioBuffer. Our rendering
pipeline has an exception for STEREO->MONO and vice versa config
changes. Any other unexpected config changes will cause a pipeline
failure. See AudioRendererImpl::DecodedAudioReady().

If instead MSE is used for demuxing, midstream config changes are
always supported, so its also not a security issue (though may
result in reduced quality in cases where the output device is
opened with the reduced channel count).

R=tguilbert

Bug: 40198159, 413913369
Change-Id: I439eed40a32634e895ae518fbad999fe9ef94c98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6495653
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452983}