Remove ButtonBacklight hardware tunable since is not used anymore

Related framework/opt/hardware commit:

    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: I8817633bfb2cf24fa22c610a5a1ee5dc18f2fbc6