mpegvideo: Drop stream_codec_tag

The field is unused.

libavcodec/mpegvideo.h

@@ -243,7 +243,6 @@ typedef struct MpegEncContext {
     int strict_std_compliance; ///< strictly follow the std (MPEG4, ...)
     int workaround_bugs;       ///< workaround bugs in encoders which cannot be detected automatically
     int codec_tag;             ///< internal codec_tag upper case converted from avctx codec_tag
-    int stream_codec_tag;      ///< internal stream_codec_tag upper case converted from avctx stream_codec_tag
     /* the following fields are managed internally by the encoder */
 
     /* sequence parameters */