Remove access to the backlight brightness

* We don't need to set the display brightness at the moment, and
  access to the brightness fd requires an SELinux policy change
  that we'd like to avoid, so ifdef out the code for now.

Bug: None
Test: To be tested along with Nick's SELinux policy cl
Change-Id: Ic6de9a1bd15d0eab2780bda31ec67fab94f8d6da