nixos/xserver: let X choose the xkb rules

With libinput used for keyboard, base rules produce incorrect keyboard
layouts. We are removing the option as recommended in the XKB configuration
guide [1] to let X server choose the ruleset. It looks like it chooses
evdev rules which seem to work for some reason

[1]: https://www.x.org/releases/X11R7.6/doc/xorg-docs/input/XKB-Config.html#id2521360