avcodec/avcodec: Remove MpegEncContext forward declaration

Forgotten in be95df12.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/avcodec.h

@@ -2029,8 +2029,6 @@ typedef struct AVCodecContext {
     int (*get_encode_buffer)(struct AVCodecContext *s, AVPacket *pkt, int flags);
 } AVCodecContext;
 
-struct MpegEncContext;
-
 /**
  * @defgroup lavc_hwaccel AVHWAccel
  *