Equipment that stops being an artifact loses the subtype “Equipment.” Permanents without the subtype Equipment can’t equip creatures. Of course, artifact creatures can’t equip anything either.

Neurok Transmuter’s second ability removes the type “artifact” — and any artifact subtypes — from the artifact creature it targets. It doesn’t remove any other types or any subtypes of other types, and it doesn’t remove supertypes. If the artifact was legendary, it’s still legendary, and the “Legend rule” applies normally.

Neurok Transmuter’s second ability interacts strangely with March of the Machines from the Mirrodin set. If an artifact is an artifact creature only because March of the Machines is on the battlefield and you then activate Neurok Transmuter’s second ability on that artifact creature, the result is a permanent with no types whatsoever. Neurok Transmuter’s ability removes the type “artifact.” March of the Machines depends on knowing what is and isn’t an artifact. The permanent won’t be an artifact when March of the Machine’s effect is applied and therefore it won’t be turned into a creature.