Try using a hacksaw and first, cut the 90 off, leaving the hub on the pipe in the wall. Then, use the blade to cut nearly through what's left in several places parallel with the pipe with maybe a 1/2-1" between the two cuts. Stick something like a screwdriver blade in the saw kerf, and see if you can split the section of fitting off. If that works, you can make more small sections, then split them off. Try not to cut into the pipe.
If you need to move the trap for the new sink, just open up the wall and don't worry about trying to save the stub.
If splitting it off doesn't work, they make special tools to ream the pipe out of the hub that is likely in the wall (RamBit is one brand). Then, you'd just glue a new stub in the cleaned out hub, and continue from there. You may want to use a Desanko (tubular pipe adapter) on the stub, and use tubular from there - it would make taking it off again simple.