preset: remove coreos presets and refer to FCOS overlay
    
    Remove presets from 80-coreos.preset, and add a note referring
    presets to be updated at the FCOS overlay RPM.
    
    This is so that there is one source of truth for presets in FCOS,
    while presets are still potentially changing.
    
    See: https://github.com/coreos/fedora-coreos-config/issues/33#issuecomment-492340243
    
    Signed-off-by: Robert Fairley <rfairley@redhat.com>