avcodec: Undeprecate reordered_opaque

It allows attaching other external, opaque data to the frame and passing it
through the reordering process, for cases when the caller wants other data
than just the plain packet pts.  There is no way to cleanly achieve this
without the field.

libavcodec/avcodec.h

@@ -2413,7 +2413,6 @@ typedef struct AVCodecContext {
     /**
      * opaque 64bit number (generally a PTS) that will be reordered and
      * output in AVFrame.reordered_opaque
-     * @deprecated in favor of pkt_pts
      * - encoding: unused
      * - decoding: Set by user.
      */