That would be the star nut which anchors the stem binder bolt and provides compressive tension to your headset bearings. They are hammered in there and they only go one way: IN further and, they are not re-usable. Let us hope that steer tube is aluminum. It is improper and unsafe to put a star nut in a carbon tube. Anyway, if you are selling the fork, the new owner will probably want the nut in there provided it is an aluminum tube. IF the nut happens to be in a carbon tube (this happens), I would say you need to disclose that fact and probably take it out. Taking one out: if the steer tube had no obstruction and did not close off at the fork, you could use a wooden dowel and a hammer to drive it all the way through. Not likely. I would say drill through the threaded hole in the center of it and then repeat with progressively larger drill bits until it collapses or relaxes so you can fish it out with a coat hanger or something. I have not had to remove one myself. Bottom line: not a bad thing unless it's in a carbon tube. Let me know what you did. Good luck.