SYSTEMD: Clean pid file in corner cases
    
    SSSD can cleanup pid file in case of standard stopping of daemon.
    It's done in function monitor_cleanup. However monitor does not have a
    change to cleanup file in case of OOM or sending SIGKILL to monitor.
    
    Even though PIDFile is not necessary for services with Type notify
    we should let systemd to clean this file in unexpected situations.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3528
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>