Disable DSA and RSA host keys
    
    DSA is obsolete. Whilst at it, remove RSA as well,
    since administrators are expected to use a modern enough client
    supporting Ed25519 or at least ECDSA.
    All internal clients and automation tools already use Ed25519.
    
    This won't remove the files on disk - doing so would require
    disabling SSHD_AUTO_KEYGEN, which is not deemed important for now
    given the low overhead.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>