I would guess that these are similar to the plastic end caps found on some flat bar type grips. It looks like there is a bit of plug extending outward around the circumference of the rack mount. I would carefully fit a flathead screwdriver behind the edge of the cap, and tap it from behind with a mallet. Once there is enough of a gap to grab, a pliers should be able to grasp and remove the cap. Just be careful not to let the screwdriver head contact the painted metal surface.