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.

slstatus.h

@@ -1,8 +1,5 @@
 /* See LICENSE file for copyright and license details. */
 
-/* backlight */
-const char *backlight_perc(const char *);
-
 /* battery */
 const char *battery_perc(const char *);
 const char *battery_state(const char *);