avcodec: remove useless palette copy
avcodec: remove useless palette copy

dec->fmt_in.video.p_palette will be used to copy to palette via
av_packet_new_side_data().

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc