You're not likely to be able to remove it without breaking it. Assuming you just want to remove it and not keep it, here's a very effective way to accomplish that.
Take a roll of duct tape and cover the entire mirror with it overlapping by about an eight of an inch each pass. Then take a hammer and break the mirror into small pieces. Wear leather gloves when you pull at the sides of the mirror. ALMOST all of the mirror will stay attached to the duct tape which can safely be deposited into the trash. The only thing left will be the spots where the mirror was glued to the wall. Simply take a scraper and scrape the glue spots from the wall and voila! mirror removed.