Fix playback rate w/o audio. Remove unused SetPlaybackRate().
NullAudioSink doesn't need to worry about playback rate since
AudioRendererAlgorithm already compresses/expands the data as
The issue is easily seen by playing any of the sync clips and
timing how fast it takes for each rotation of the "clock" in
the video. Setting playback rate to 2 will actually end up
being 4x as fast currently.
As a bonus, dropping this allows us to remove the now unused
TEST=Play sync clip, time "clock" rotation at various speeds.