Are you planning to reinstall it, replace it or fill in the hole?
Fill in the hole: Cut the drywall around the soap-dish and remove the whole shebang. The drywall patch will be a little larger, but so what?
Replace it: Cut the existing dish up with tin snips and remove it in pieces. Could be a pain in the ass depending on the thickness of the metal. You might get lucky. If you use a larger dish than the existing one, you could combine the first these first two methods.
Reinstall it: Do you even have to remove it? You might be able to glue it back in place and cut the wallpaper around the dish. Use stronger adhesive around the cut edges to keep them from curling up in the moist air of the bathroom.
R