lavfi/gradfun: remove nv21/nv12 as they are not supported

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_gradfun.c

@@ -149,8 +149,7 @@ static int query_formats(AVFilterContext *ctx)
 {
     static const enum AVPixelFormat pix_fmts[] = {
         AV_PIX_FMT_YUV410P,            AV_PIX_FMT_YUV420P,
-        AV_PIX_FMT_GRAY8,              AV_PIX_FMT_NV12,
-        AV_PIX_FMT_NV21,               AV_PIX_FMT_YUV444P,
+        AV_PIX_FMT_GRAY8,              AV_PIX_FMT_YUV444P,
         AV_PIX_FMT_YUV422P,            AV_PIX_FMT_YUV411P,
         AV_PIX_FMT_YUV440P,
         AV_PIX_FMT_NONE