Clean or Replace subfloor
Replacing the subfloor will not be easy, it likely runs under the walls.  You'll have to cut around the perimiter and the pry up the boards which are likely gluded down as well. 

If you want the glue removed try a belt sander with a 40 grit paper or lower and see how well that removes the glue.  You'll gum up a lot of belts, but the area is still small. Wear a dust mask.