MONITOR: Remove the no longer used diag_cmd command
    
    After introducing the watchdog, the diag_cmd is longer used and makes no
    sense trying to make it usable by watchdog as the result of "pstack %p"
    seems next to useless in this context.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3051
    
    Reviewed-by: Petr Čech <pcech@redhat.com>