XFA: remove unused GdiCombineMode2Gdip() [windows]
XFA: remove unused GdiCombineMode2Gdip() [windows]

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1428713004 .

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