<p>You don't even have to notify them if you don't want to, although that would certainly be polite. The stuff your GC sent to XYZ will be put in a file to await completion of your application. The XYZ adcom won't review it unless the application is complete. If you don't submit the Common App forms, your application won't be complete. So the worst thing that will happen to you is you'll get some junk mail (and/or email) from XYZ telling you that your application is incomplete.</p>