Usually a single-edged razor will take them up. (Those "Wdigets" by Gillete are handy, or you can get a scraper into which you slide a razor.) Use a solvent remover like Goof Off or Goo Gone to remove the remaining adhesive. If the razor does not work on its own, then use the solvent before the razor, and it should come up.

Manufacturer (3M) recommends peeling the nonskid layer and the next mylar layer off the 'decal'.......then use a laundry presoak to remove sticky glue residue......doug

Try using a hair dryer and a rubber spatula to scrape them off the tub.