Remove _FPDF_ByteStringFromHex as it is unused
Remove _FPDF_ByteStringFromHex as it is unused

There are no uses of this method, I also checked the XFA branch and there don't
appear to be any uses there either.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1427913006 .

andoma/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,andoma/pdfium,DrAlexx/pdfium,andoma/pdfium,andoma/pdfium,DrAlexx/pdfium