af_hdcd: disable auto-convert by default

As all known valid HDCD sample formats and sample rates are now handled
by the filter, remove the scan that "invades the privacy" of the filter graph
and turn off autoconvert by default as requested by Nicolas George.

http://ffmpeg.org/pipermail/ffmpeg-devel/2016-August/197571.html

Signed-off-by: Burt P <pburt0@gmail.com>
