[IMP] account_payment_order: No error on unknown payment method codes

Since v14, the tool for changing the target transfer account is to
create new payment methods, and put a method line in the journal with
such account.

Thus, there will be "fake" payment methods with no export handler
with the only goal of having a different target account.

Raising an error on such methods when finishing the payment order, forcing
to add custom code for returning (False, False), is too much, so better
to simply return that by default and don't block in these cases.

TT47723

account_payment_order/models/account_payment_order.py

@@ -376,17 +376,13 @@ class AccountPaymentOrder(models.Model):
         return True
 
     def generate_payment_file(self):
-        """Returns (payment file as string, filename)"""
+        """Returns (payment file as string, filename).
+
+        By default, any method not specifically intercepted by extra modules will do
+        nothing, including the existing manual one.
+        """
         self.ensure_one()
-        if self.payment_method_id.code == "manual":
-            return (False, False)
-        else:
-            raise UserError(
-                _(
-                    "No handler for this payment method. Maybe you haven't "
-                    "installed the related Odoo module."
-                )
-            )
+        return (False, False)
 
     def open2generated(self):
         self.ensure_one()