Get a hammer and a scraper like a 5 way tool or a small chisel. Just remove the thinset but leave the cement board. it wont be easy because you dont have a lot of room to work in the opening but it looks like its not an overabundence of thinset to deal with. There are specialty tools that would work great but not the kind of thing a rental center would carry so unless you know someone who owns one, the old method will have to do. Its Ok if some of the top layer of cement board comes off but you dont want a gaping hole there, the soap dish needs a solid backing for the thinset to bite and hold while it dries. Dont use glue or mastic, rather use modified thinset that comes in a bag and mixes with water. Use tape to hold the dish in place while it dries overnight.