Remove an obsolete comment for onAfterDraw in pdf_viewer.js

With the viewer code now being split into various components/files, having an obsolete comment in `PDFViewer` that references thumbnails despite there being no other mentions of them in the entire file seems strange.

*Note:* This comment is simply a left-over from older versions of PDF.js, where the *entire* default viewer code was placed in just one file (and where we unconditionally created thumbnails, regardless whether they were visible or not).

web/pdf_viewer.js

@@ -323,7 +323,6 @@ var PDFViewer = (function pdfViewer() {
           // rendering.
           self._buffer.push(this);
         };
-        // when page is painted, using the image as thumbnail base
         pageView.onAfterDraw = function pdfViewLoadOnAfterDraw() {
           if (!isOnePageRenderedResolved) {
             isOnePageRenderedResolved = true;