clang/win: Stop passing -Qunused-arguments in pdfium too.
clang/win: Stop passing -Qunused-arguments in pdfium too.

We stopped passing this in Chromium a while ago.

BUG=504658

Review URL: https://codereview.chromium.org/1887643002

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