You can use a small drill bit to gently drill into and pull out the corncob. I would do it by hand with a pin vice holding the drill bit. Just be darn sure to wear eye and ear protection and a glove while doing it in case a primer goes off. If the drill bit makes it all the way to the bottom of the primer anvil, it could also disturb the priming mix and cause a squib, so don't use this ammo for anything you need to rely on, like match ammo.
Short of that, if you are using a fast pistol powder in this, the chances are the corncob will just blow out of the gun along with other unburned debris. I would also recommend against using rounds loaded this way for match ammo, both because of the corncob possibly causing short ignition delays that tend to let shooter wobble open up groups, and because the tumbler-vibrated primers have more chance of producing a squib, though it probably won't be much.
For the future, as others commented, do your cleaning before you size and prime. That improves the life of your sizing die and lets your decapping pin push the corncob out. As suggested, go to a smaller size of corncob or the walnut to avoid plugged flash holes.
