avcodec/mpegvideo_enc: Don't set picture_in_gop_number for slice threads

They don't ever read this value.

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

libavcodec/mpegvideo_enc.c

@@ -260,7 +260,6 @@ static void update_duplicate_context_after_me(MpegEncContext *dst,
     COPY(qscale);
     COPY(lambda);
     COPY(lambda2);
-    COPY(picture_in_gop_number);
     COPY(frame_pred_frame_dct); // FIXME don't set in encode_header
     COPY(progressive_frame);    // FIXME don't set in encode_header
     COPY(partitioned_frame);    // FIXME don't set in encode_header