avcodec/mfenc: remove usage of avctx->pkt_timebase

The field is documented to be for decoding only.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/mfenc.c

@@ -100,8 +100,6 @@ static int mf_wait_events(AVCodecContext *avctx)
 
 static AVRational mf_get_tb(AVCodecContext *avctx)
 {
-    if (avctx->pkt_timebase.num > 0 && avctx->pkt_timebase.den > 0)
-        return avctx->pkt_timebase;
     if (avctx->time_base.num > 0 && avctx->time_base.den > 0)
         return avctx->time_base;
     return MF_TIMEBASE;