Opposite of Hoarder

One article describes it as a mental illness: obsessive-compulsive spartanism. That's fairly negative, but you might not want the added baggage of diagnosing someone with a mental illness, so just spartanism might be a good alternative. Spartanism is a little more neutral to me than negative, but some folks might see it as slightly negative.

An article in the Atlantic - The Opposite of Hoarding discusses the challenges faced by folks with this specific flavor of compulsive disorder, because of the cultural view of 'decluttering' as a positive thing. It says "...there’s no official term for compulsive decluttering."

People who have (or want) few possession are called minimalists. People can "travel light" by not carrying possessions with them.