liblights: Fix variable button brightness code

* Because we're using #ifdef in set_light_buttons(),
  defining VAR_BUTTON_BRIGHTNESS to anything, including 0,
  results in the button brightness code being enabled
* That results in broken touchkey backlight for devices
  that don't override the samsung_lights header, or defined
  this macro to 0 in their device's samsung_lights header
* Comment out the macro to correct this logic

Change-Id: Icf554b0deac975f729f1a70e517341dae74f3657