Deletes a custom field definition in KlickTipp and with it every value the contacts of the account hold in that field. This destroys contact data and cannot be undone, so only call it for a field the user has explicitly asked to delete. Only fields the account created itself can be deleted; the global fields every KlickTipp account has are refused. A field that opt-in forms, automations or other entities still use is refused as well, and the refusal names them, so nothing that reads the field breaks silently. The placeholder of a deleted field renders an empty value wherever content still carries it.