avfilter/af_asetnsamples: remove .needs_writable as it is not required

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

libavfilter/af_asetnsamples.c

@@ -170,7 +170,6 @@ static const AVFilterPad asetnsamples_inputs[] = {
         .name           = "default",
         .type           = AVMEDIA_TYPE_AUDIO,
         .filter_frame   = filter_frame,
-        .needs_writable = 1,
     },
     {  NULL }
 };