clevis: simplify sysusers.d fragment by using default 'nologin' shell
    
    This tweaks the existing sysuser.d fragment in order to simplify it.
    The 'nologin' shell is the documented systemd default, so there is
    no need to explicitly specify it.
    This change allows better handling of default vs custom shell in the
    macro logic which bridges between `systemd-sysusers` and `useradd`.