Remove 2 unused variables from MPV_decode_mb_internal() found by CSA.

Originally committed as revision 18559 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -1780,16 +1780,12 @@ static av_always_inline
 void MPV_decode_mb_internal(MpegEncContext *s, DCTELEM block[12][64],
                             int lowres_flag, int is_mpeg12)
 {
-    int mb_x, mb_y;
     const int mb_xy = s->mb_y * s->mb_stride + s->mb_x;
     if(CONFIG_MPEG_XVMC_DECODER && s->avctx->xvmc_acceleration){
         ff_xvmc_decode_mb(s);//xvmc uses pblocks
         return;
     }
 
-    mb_x = s->mb_x;
-    mb_y = s->mb_y;
-
     if(s->avctx->debug&FF_DEBUG_DCT_COEFF) {
        /* save DCT coefficients */
        int i,j;