How to remove the "Second name" field in the billing form, I can not find the path to reach this form and modify it. Thanks
Unfortunately you cannot control billing block fields through UI. There is no such settings since this fields are mandatory for some(infact many) payment processors. This fields are hard coded at https://github.com/civicrm/civicrm-core/blob/master/CRM/Core/Payment.php#L869
You can use buildform hook to remove the second name field.
Note: You might get error from some payment processor if second name is empty