Removing a glued down wood stair tread
Unfortunately if it was glued properly and has fully dried it may be almost impossible to get the tread up. Wood glue is extremely strong (it can actually be even stronger than the bond between the wood fibers).

I don't know what to suggest other than going at it with a demo pry bar. If you are able to get most of it up you can clean up the plywood with a chisel and/or a belt sander.