doc/encoders: Fix libkvazaar documentation

The -threads option is ignored with libkvazaar since it does not have
any of the AV_CODEC_CAP_{FRAME,SLICE,AUTO}_THREADS capabilities. This
commit removes the incorrect documentation as well as the no-op of
setting the number of threads in libkvazaar encoder.
Signed-off-by: Arttu Ylä-Outinen <arttu.yla-outinen@tut.fi>

doc/encoders.texi

@@ -2401,9 +2401,6 @@ configuration. You need to explicitly configure the build with
 @item b
 Set target video bitrate in bit/s and enable rate control.
 
-@item threads
-Set number of encoding threads.
-
 @item kvazaar-params
 Set kvazaar parameters as a list of @var{name}=@var{value} pairs separated
 by commas (,). See kvazaar documentation for a list of options.

libavcodec/libkvazaar.c

@@ -74,7 +74,6 @@ static av_cold int libkvazaar_init(AVCodecContext *avctx)
     cfg->height = avctx->height;
     cfg->framerate =
       avctx->time_base.den / (double)(avctx->time_base.num * avctx->ticks_per_frame);
-    cfg->threads = avctx->thread_count;
     cfg->target_bitrate = avctx->bit_rate;
     cfg->vui.sar_width = avctx->sample_aspect_ratio.num;
     cfg->vui.sar_height = avctx->sample_aspect_ratio.den;