NSS: Don't call chown on NSS service's ExecStartPre
    
    The sssd-nss.service attempts to chown its log file to ensure it has the
    correct owner. Unfortunately, when this happens, it enters in a loop
    trying to call into the name-service switch and hangs forever.
    
    For now the approach taken is to just remove the ExecStartPre from the
    NSS service.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3322
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>