avutil/pixdesc: remove unneeded assignment

Found-by: CSA
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/pixdesc.c

@@ -2263,7 +2263,6 @@ enum AVPixelFormat av_find_best_pix_fmt_of_2(enum AVPixelFormat dst_pix_fmt1, en
     if(!has_alpha)
         loss_mask &= ~FF_LOSS_ALPHA;
 
-    dst_pix_fmt = AV_PIX_FMT_NONE;
     score1 = get_pix_fmt_score(dst_pix_fmt1, src_pix_fmt, &loss1, loss_mask);
     score2 = get_pix_fmt_score(dst_pix_fmt2, src_pix_fmt, &loss2, loss_mask);