Clean up unused code in ViewportArguments.h
https://bugs.webkit.org/show_bug.cgi?id=115121

Patch by Jacky Jiang <zhajiang@blackberry.com> on 2013-04-24.
Reviewed by Benjamin Poulain.

The target-densityDPI emulation implementation has been removed by
r147893, so clean up the remaining code as well.

* dom/ViewportArguments.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149062 268f45cc-cd09-0410-ab3c-d52691b4dbfc