gluing mirrors to a wall is fairly common. I have taken them off using a thin putty knife to start. Start on one corner, initially you are just trying to get a corner lifted away from the glue. Get some braided fishing line (50-80lb test) and slide it under the putty knife and the corner of the glass, remove the putty knife, and start sawing the line back and forth. Some glues will stick to the line so move to a section of fresh line as required. I also should tell you that success is not always the outcome, some times the mirror breaks, it is a good idea to put some sticky paper on the mirror so that if it does break the glass is contained. I use that stuff from home depot that you line shelves and drawers with. You will (most likely) have wall repairs to do after removal as well, certainly glue residue, depends on what the wall material is as to how you remove it, some solvents will affect wallpaper.