Revert "Merge pull request #927 from enyojs/ENYO-451-mikyungkim"

This reverts commit 18fd7cf70a42ba40ebf9d00c30d5bf64eaca2d2b, reversing
changes made to 3b6f1870b50bfe943bd935fdbc9836e9fd4bff53.

This change is not needed with the Page Visibility API.
https://github.com/enyojs/enyo/blob/master/source/dom/page_visibility.js