In the traditional chicken house with concrete floor, it did not have drains. The slab itself was poured on top of the foundation footings, with a gravel back fill beneath it. The gravel backfill also had drain tile in it (under the slab) to eliminate any moisture from coming up from below. They did not use a moisture barrier, but you could. Essentially, the floor was elevated above the surrounding dirt, so the elevation kept it dry. They wanted their concrete floor dry as a bone.
Either they did not have the resources to flush/hose it out, or never saw the need to. Most moisture created by the birds went out the door with the litter, or if well ventilated as it's supposed to be, evaporated and drifted out the window.