Revert "Button & keyboard backlight brightness adjustment (1/2)"

This reverts commit 281b4045e0baa18fcc5001f395ef39940048cd96, since
backlight can and should be managed by the standard liblights support
in PowerManager. Support for that was just added to Settings

Change-Id: Ia6e6258a05222e2f63099febc71beeb345f0ad89