Stop including out-of-tree modules

    Stop including out-of-tree kernel modules
    
    This presents some practical problems:
    - We'll almost certainly fail shim review and therefore won't be able to
      have secure boot
    - Pre-installing these modules taints our kernel, which will be
      problematic for engagement with upstream
    - These modules can be fragile (see for example
      https://invent.kde.org/kde-linux/kde-linux/-/work_items/618)
    - Including these modules broadens the attack surface, worsening
      security.
    
    As nice as it is to have an "everything and the kitchen sink" approach
    to hardware support via these out-of-tree kernel modules, I think the
    drawbacks to the project as a whole and all of its users outweigh the
    benefits to the specific people who benefit here.
    
    Fixes #618

    Remove DKMS machinery
    
    We can't use DKMS to load out-of-tree modules into our kernel by default
    without incurring negative consequences we don't want to deal with.
    Remove the machinery for now since we won't be using it.
    
    Reverts 7f3f03a19fa7801d8f4bf78e3ba6c809dc56b620
    
    Related to #3