rpenmanparker is correct

the top smooth cylinder is a threadless stem adapter
it is held in by an expander or wedge held in by a bolt that runs up to the top

there is probably a bolt that can be accessed through the top
you probably need a 6mm allen key to loosen it
and possibly some way to give it a blow from a hammer to free the wedge underneath
then the adapter will slide out
and the threaded headset components can be removed
and the fork will come out