Update notifications.js

Regarding the bug no. 1006990 .
Gaia apps was loaded with app cache, and we have switched to packaged app long time ago. However, there are still redundant code in the top of apps/system/js/notification.js on saving notification icons.
So removing the app cache code.
Merge pull request #22702 from AkshayTiwari/master

Bug 1006990 - Remove broken applicationCache handling in notification.js, r=timdream