Our own code was introduced when FFmpeg didn't provide this API (or
maybe didn't even have a way to determine the CPU count). But now,
av_cpu_count() is available for all FFmpeg/Libav versions we support,
and there's no reason to have our own code.
libavutil's code seems to be slightly more sophisticated than our's, and
it's possible that the detected CPU count is different on some platforms
after this change.
d7dfbc8610