Can I temporarily remove the bottom stairs on a staircase to help move furniture?
They are what is know to me as "housed stringers" the sides, have notches cut into them to allow the treads to set in the notches. The small vertical pieces in the corners are wedges driven in with glue to snug everything up tight to reduce or eliminate squeaks. To remove the treads, you would need to cut out the treads, and install new ones... Not what you may want to read, but it is what it would take. 

For what its worth, if there were no glue involved, the wedges could be removed, the nails pulled and the treads could be pulled out of the slots from the underside, if there are no nails in from the sides covered by the walls.