remove readahead ordering hacks
    
    The dependency cycle this introduced can cause systemd to get stuck
    under some circumstances.
    
    We will just disable systemd-readahead in the kickstart instead, because
    getting this ordering hack to work properly in all cases is just not
    feasible.
    
    Bug: 1147807
    Change-Id: I517c331c203981460e8440ac14c8b279c9f9c5b1