avcodec/mpv_reconstruct_mb_template: Optimize dead code away

None of the MPEG-1/2 codecs support frame threading,
so one can optimize the check for it away.

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

libavcodec/mpv_reconstruct_mb_template.c

@@ -121,7 +121,8 @@ void mpv_reconstruct_mb_internal(MpegEncContext *s, int16_t block[12][64],
             /* decoding or more than one mb_type (MC was already done otherwise) */
 
 #if !IS_ENCODER
-            if (HAVE_THREADS && s->avctx->active_thread_type & FF_THREAD_FRAME) {
+            if (HAVE_THREADS && is_mpeg12 != DEFINITELY_MPEG12 &&
+                s->avctx->active_thread_type & FF_THREAD_FRAME) {
                 if (s->mv_dir & MV_DIR_FORWARD) {
                     ff_thread_await_progress(&s->last_picture_ptr->tf,
                                              lowest_referenced_row(s, 0), 0);