lavc: update coded_frame doxy.

It is not used during decoding anymore.

libavcodec/avcodec.h

@@ -2496,7 +2496,7 @@ typedef struct AVCodecContext {
     /**
      * the picture in the bitstream
      * - encoding: Set by libavcodec.
-     * - decoding: Set by libavcodec.
+     * - decoding: unused
      */
     AVFrame *coded_frame;