lavc: stop setting AVFrame.motion_subsample_log2

It is not used inside lavc anywhere and now it makes no sense to export
it.

libavcodec/error_resilience.c

@@ -851,7 +851,6 @@ void ff_er_frame_end(ERContext *s)
             }
             return;
         }
-        s->cur_pic->f.motion_subsample_log2 = 3;
     }
 
     if (s->avctx->debug & FF_DEBUG_ER) {

libavcodec/h264.c

@@ -360,7 +360,6 @@ static int alloc_picture(H264Context *h, Picture *pic)
         pic->motion_val[i] = (int16_t (*)[2])pic->motion_val_buf[i]->data + 4;
         pic->ref_index[i]  = pic->ref_index_buf[i]->data;
     }
-    pic->f.motion_subsample_log2 = 2;
 
     return 0;
 fail:

libavcodec/svq3.c

@@ -1058,7 +1058,6 @@ static int get_buffer(AVCodecContext *avctx, Picture *pic)
             pic->ref_index[i]  = pic->ref_index_buf[i]->data;
         }
     }
-    pic->f.motion_subsample_log2 = 2;
     pic->reference = !(h->pict_type == AV_PICTURE_TYPE_B);
 
     ret = ff_get_buffer(avctx, &pic->f,