Fix for bug #9231: B-frames parameter is ignored in videotoolboxenc

This commit fixes the bug as report in
https://trac.ffmpeg.org/ticket/9231 by removing the line that
overwrites the user settings for max_b_frames.

Signed-off-by: Anselm Busse <anselm.busse@outlook.com>
Signed-off-by: Rick Kern <kernrj@gmail.com>

libavcodec/videotoolboxenc.c

@@ -1398,7 +1398,6 @@ static int vtenc_configure_encoder(AVCodecContext *avctx)
     }
 
     vtctx->codec_id = avctx->codec_id;
-    avctx->max_b_frames = 16;
 
     if (vtctx->codec_id == AV_CODEC_ID_H264) {
         vtctx->get_param_set_func = CMVideoFormatDescriptionGetH264ParameterSetAtIndex;