Revert "Add basic backlight percentage support"

Revert "Add basic backlight percentage support"

On OpenBSD the backlight percentage cannot be retrieved in a simple way.
The only two solutions we are aware of for now are:

 - reading from /dev/ttyC0: which isn't possible without changing
 permissions or running slstatus as root
 - linking against xcb-xrandr: which is bloat and does not work in every
 case appearently

This reverts commit 37724ac2c3f496f4736223d0d8e5d8fecb933590 for now.