Remove unused mw.notify msg keys from ext.visualEditor.core module

These 3 mw-notify message keys:
* visualeditor-notification-saved
* visualeditor-notification-created
* visualeditor-notification-restored

Are only used by ve.init.mw.ViewPageTarget.js, which is part
of ext.visualEditor.viewPageTarget, not ext.visualEditor.core.

Change-Id: Ib094c0a2934856b90349a9f8890a20feebf0837c
Merge "Remove unused mw.notify msg keys from ext.visualEditor.core module"