validate_principal: Don't try to verify that the realm is known
    
    The actual value is less important than whether it matches the
    regular expression. A number of legal but difficult to know in
    context realms could be passed in here (trust for example).
    
    This fixes CVE-2024-1481
    
    Fixes: https://pagure.io/freeipa/issue/9541
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>