The ThingDef is in ...\RimWorld\Mods\Core\Defs\ThingDefs_Buildings\Buildings_Natural.xml
If you change the line <destroyable>false</destroyable> to true and then start the game, you can remove the steam geyser.
If you change the line <destroyable>false</destroyable> to true and then start the game, you can remove the steam geyser.