The best way to remove mirror tiles from a wall is to use a wallpaper scraper or filling knife. Wearing protective eye goggles and gloves, put the tool behind the tile, to stop the tile splintering put a cloth or towel against it, and prize it away as best you can. To get the wall smooth try using white spirit to get off any old adhesive left underneath, or warm it with a hair dryer or professional heat gun so that it is easier to remove.