An air chisel with a with a wide blade has been the best for me, the above multi tool with the right blade works but is slower, and a large grinder with the coarsest sanding
disk you can find. Sometimes the sanding
disk of lighter grit will work
under non skid and lift
it. All of it depends on how old and brittle the old non skid is.
I prefer the old coarse sand and paint
non skid. It last longer, but is tough on shoe soles and bare feet.