Specialized Rock Hopper (1987) - Service
These are just old U-Brakes (installed at the chain stays as was cool for part of the late 80s). You just need to unbolt them from the studs (maybe hit it with some penetrating oil to help loosen it oFF the studs). Unfortunately due to the placement of the U brake , you're going to need to get another u brake since you won't be able to mount a normal brake that way.