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 37724ac2c3 for now.

config.def.h

@@ -12,8 +12,6 @@ static const char unknown_str[] = "n/a";
 /*
  * function             description                     argument (example)
  *
- * backlight_perc       backlight percentage            device name
- *                                                      (intel_backlight)
  * battery_perc         battery percentage              battery name (BAT0)
  *                                                      NULL on OpenBSD
  * battery_state        battery charging state          battery name (BAT0)