Apply an adhesive removing paste to the wall.
How to remove tile adhesive off walls.
Look for a formula that is either made for ceramic tile.
Use firmer pressure and more steam if not enough of the adhesive comes off.
Place the anvil close to the wall and allow the steam to penetrate the adhesive for five minutes.
Apply some water to a small section of the adhesive.
Use a paper towel and gently scrub the area in a circular motion.
Clumps of tile adhesive divots from where part of the drywall came off with the tile and other marks will probably be left on the wall as well.
Give a few taps until the tile adhesive becomes loose enough to fall off or be scraped off with the end of the chisel.
It will not be a easy task to remove the old adhesive you can tile on top of the old one but what i am suggesting is to level the walls with tile adhesive or with adhesives with cement in a there composition and not with bonding compounds as it is not water resistant and being a bathroom a good strong support is recommended.
Irrespective of the adhesive formula you have to cover the floor with old cloths as to protect it from the residues which will fall off the wall.
Wait several minutes and use a large scrapper to remove the mastic.
Tiles installed with thinset or another tile adhesive often require more work to remove them.
If the adhesive is not cement based you could try sprinkling water to the wall to ease the scrapping process.
Use your hammer to gently tap at the end of the chisel to pry off the adhesive.
If the adhesive used was organic mastic the water will begin to soften it allowing you to.
The method used to attach the tiles to the wall affects the way the tiles come off.
Chisel large chunks of tile adhesive off the walls.
Place the chisel at the edge of the piece of adhesive holding it at about a 15 degree angle.