I need to delete the Apple ID of my dads iPad. He died and I don't know his password.
As far as I know the only way to remove an iCloud password is to contact Apple with the relevant documentation.
This link has some contact details
But if you want to transfer the account call Apple Support. You'll need the death certificate and a court document that confirms that you are the decedent's legal personal representative. That document needs to be notarized or certified by a US court.
If you just want to remove the account from the iPad, if you have a receipt or invoice from where the device was purchased with the devices serial/IMEI number along with the sellers location, and contact info then the FindMyiPhone team at Apple Support can remove the account from the iPad.
If you can't do either, then at least call in and get help with recovering the password by calling in. Worst case scenario, it'll put you in account recovery in which after waiting a period of time, you'll get access to the account.