It is called a brake boss and that looks like a sleeve from the inside of a V-brake or Linear brake. Squirt it with some penetration oil and then use a small pair of channel lock pliers to twist it off. It also looks like your brake bosses are replaceable, I think I see flats to get them our, most likely a 9mm open end wrench would work, may have to heat it up first as there is normally thread locker of the bosses threads.