configure: remove old libdcadec check

It's obsolete after the addition of the pkg-config check.
See http://comments.gmane.org/gmane.comp.video.ffmpeg.devel/191983 for the
relevant discussion

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>
