OmniGears: cleanup duplicate button brightness setting

Currently, button brightness settings are accessible from 2 locations:

Settings > Display > Brightness

and

Settings > Buttons

Since it's BUTTON settings, let's contain these settings to the BUTTONS category

Change-Id: I4164791d902a750b2ee3c6cf8ebdeddd4162db99
Merge pull request #16 from omnirom/android-4.4

OmniGears: cleanup duplicate button brightness setting