How do you get the glue of the walls once you take the mirrors off.?

If these mirrors were up for a many years you are probably not going to be able to use a solvent to remove the adhesive. They were probably stuck up there with a construction adhesive.

You might have to use a putty knife to scrape it off. There is a good chance the paper on the sheetrock will come off with the adhesive.

When you are done you can put on a thin coat of drywall mud (joint compound) after it dries lightly sand it.

Bathroom mirrors are indeed typically glued to the drywall. There is no way to remove it without damage to the wall behind The glue is in "globs"; the entire surface is not glued. So the objective is to get each one of those spots of glue to break loose from the wall without breaking the glass. Keep in mind such mirrors are not safety glass; they can break into deadly shards. You may consider applying strips of duct tape to the mirror in case it should break. Starting at an accessible corner, use a pry bar to carefully pull the glass from the wall.Apply just enough pressure until you feel the glue is pulling the paper off the drywall. Then move a few inches over and pry again. Continue this moving across the sides of the mirror. When you get the glass off, you have to patch the wall; but that is another question.

Use a razor type of scraper. It has hardened by now. You may need a little filler to smooth the gouging that will occur while removing the glue. Best of luck

Try Goo Gone

It's good at getting most sticky things (like residue from old tape, stickers etc) off of surfaces. I got mine at Home Depot (it's cheap), and I'm sure it is available in lots of other stores.

heavy duty paint scraper & some elbow grease

Try using nail polish remover or Avon Skin So Soft lotion.

I use lighter fluid

take it off wit a screwdriver.