Remove deprecated date.js library

This library has been over-writing methods in Date.prototype which is a discouraged practise in shared JavaScript environments.

The only usage was the Date.parseExact function and that too only for parsing the yyyyMMDDHHmmss format. This functionality has been implemented in the newly added ext.pageTriage.dateUtil.js file. There was also one usage of the same function for parsing the yyyy-MM-ddTHH:mm:ssZ format. This is a standard ISO format for which new Date() suffices in all browsers.

Bug: T268513
Change-Id: Ie2f192ec4f8503db89d7fa26de812a3d3914b068