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>