Use grayscale font anti-aliasing

As far as I'm aware it's currently not possible to use grayscale font anti-aliasing, only sub-pixel rendering.

I tried xfconf-query -c xsettings -p /Xft/Lcdfilter -n -t string -s lcdnone but this results in even heavier sub-pixel rendering. Setting the sub-pixel order to None in the appearance settings also didn't disable sub-pixel rendering.