That is a threaded headset and the stem should have a stem bolt.

My guess is that that wedge in the stem is stuck (or the stem is seized) and one would normally use the stem bolt to tap out the wedge, I'd start by locating a stem bolt and seeing if it can be threaded back in and from there one can tap on that to loosen the wedge.

Technique is to thread the stem bolt in until it is snug and then back it out 4-5 turns and give it a firm tap with a hammer to free the wedge... if it is not seized the stem should then be freed up for removal and then you can address the headset.