如何使用itextsharp C#从签名字段中删除有效性未知图标？

PdfReader reader = new PdfReader("path/to/input.pdf");

AcroFields fields = reader.AcroFields;
List<string> signatureNames = fields.GetSignatureNames();

foreach (string signatureName in signatureNames)
{
    PdfDictionary signatureDict = fields.GetSignatureDictionary(signatureName);
    PdfName subFilter = signatureDict.GetAsName(PdfName.SUBFILTER);
    
    if (subFilter.Equals(PdfName.ADBE_PKCS7_DETACHED))
    {
        int sigFlags = signatureDict.GetAsNumber(PdfName.SIGFLAGS)?.IntValue ?? 0;
        
        if ((sigFlags & 1) == 0) // Check if validity is unknown
        {
            PdfArray annots = reader.AcroFields.GetFieldItem(signatureName).GetWidgetRef().GetAsArray(PdfName.ANNOTS);
            
            if (annots != null)
            {
                foreach (PdfObject annotObj in annots)
                {
                    PdfDictionary annotDict = (PdfDictionary)PdfReader.GetPdfObject(annotObj);
                    PdfName annotSubtype = annotDict.GetAsName(PdfName.SUBTYPE);
                    
                    if (annotSubtype.Equals(PdfName.WIDGET))
                    {
                        PdfDictionary apDict = annotDict.GetAsDict(PdfName.AP);
                        apDict.Remove(PdfName.N);
                        apDict.Remove(PdfName.DN);
                    }
                }
            }
        }
    }
}

PdfStamper stamper = new PdfStamper(reader, new FileStream("path/to/output.pdf", FileMode.Create));
stamper.Close();
reader.Close();