Remove completely unused fontScale property from PageViewport

The `fontScale` property was added in PR 1531, see commit b312719d7e in particular, apparently for the sole purpose of supporting the "acroforms" example.

However, the `fontScale` property was never used anywhere else in the code-base, and after the modernization of the "acroforms" example in PR 8030 it's been completely unused.

Finally, note that there's also a (more suitably named) `scale` property on `PageViewport` instances, which contains the exact same information as the property being removed here.

src/display/dom_utils.js

@@ -212,7 +212,6 @@ var PageViewport = (function PageViewportClosure() {
 
     this.width = width;
     this.height = height;
-    this.fontScale = scale;
   }
   PageViewport.prototype = /** @lends PageViewport.prototype */ {
     /**