How to remove mastic from hardibacker

I am remodeling my bathroom. Years ago I was told to always use mastic for vertical tile, so I bought a premium mastic and began the shower tile over hardibacker. I am always looking up videos and DIY helps and discovered that mastic is the wrong material to use in a wet area. I had only laid 2 rows of tile on one wall of the shower and am removing them. Since learning that mastic is an organic material and all it needs is to have water introduced to it to promote mold, can anyone tell me how to remove all the mastic before introducing thinset? I have been removing with a chisel, but obviously there is a lot of residue.