This will take a while, but it will work. Just find a bucket and toss those puppies in with warm water and liquid dish soap. Check again next weekend and see if that thinset is ready to be brushed off (usually with a regular scrub brush). Perhaps some of the guys on this site know a more "professional" process/chemical that will work here (I know the do make a thinset remover), but this is cheap and easy if you're not in too big a hurry. Good luck.