avfilter/vf_libplacebo: don't force dovi frames to bt.2020-ncl

This is at odds with the YUV matrix negotiation API, in which such
dynamic changes in YUV encoding are no longer easily possible. There is
also no really strong motivating reason to do this, since the choice of
YUV matrix is essentially arbitrary and not actually related to the
Dolby Vision decoding process.

libavfilter/vf_libplacebo.c

@@ -888,7 +888,6 @@ static int output_frame(AVFilterContext *ctx, int64_t pts)
     if (s->apply_dovi && av_frame_get_side_data(ref, AV_FRAME_DATA_DOVI_METADATA)) {
         /* Output of dovi reshaping is always BT.2020+PQ, so infer the correct
          * output colorspace defaults */
-        out->colorspace = AVCOL_SPC_BT2020_NCL;
         out->color_primaries = AVCOL_PRI_BT2020;
         out->color_trc = AVCOL_TRC_SMPTE2084;
     }