avcodec/decode: remove unused AVCodecInternal compat_decode field

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/decode.c

@@ -736,7 +736,6 @@ static int compat_decode(AVCodecContext *avctx, AVFrame *frame,
     }
 
     *got_frame = 0;
-    avci->compat_decode = 1;
 
     if (avci->compat_decode_partial_size > 0 &&
         avci->compat_decode_partial_size != pkt->size) {

libavcodec/internal.h

@@ -173,8 +173,6 @@ typedef struct AVCodecInternal {
     int buffer_pkt_valid; // encoding: packet without data can be valid
     AVFrame *buffer_frame;
     int draining_done;
-    /* set to 1 when the caller is using the old decoding API */
-    int compat_decode;
     int compat_decode_warned;
     /* this variable is set by the decoder internals to signal to the old
      * API compat wrappers the amount of data consumed from the last packet */