Remove redundant TODO

In iText 5 ltv verification was a part of PdfStamper's functionalty.
However, in iText7 we do have a separate class for it.

DEVSIX-5142

Author: ars18wrw

sign/src/main/java/com/itextpdf/signatures/PdfSigner.java

@@ -872,9 +872,6 @@ protected void preClose(Map<PdfName, Integer> exclusionSizes) throws IOException
         if (preClosed) {
             throw new PdfException(PdfException.DocumentAlreadyPreClosed);
         }
-
-        // TODO: add mergeVerification functionality
-
         preClosed = true;
         PdfAcroForm acroForm = PdfAcroForm.getAcroForm(document, true);
         SignatureUtil sgnUtil = new SignatureUtil(document);