uba.remove_item_conf = function(arena_name,name,itemstring)
print("itemstring "..dump(itemstring))
	local conf_file = Settings(uba.mod_path.."/"..arena_name.."/items.conf")
	print("path "..dump(uba.mod_path.."/"..arena_name.."/items.conf"))
	if conf_file:remove(itemstring) then
		conf_file:write()
		minetest.chat_send_player(name,"* "..itemstring.." removed from "..arena_name.."/items.conf")
	else
		minetest.chat_send_player(name,"*Could not remove unknown item")
	end
end

> console output
> itemstring "default:sword_diamond"
> path "/home/raphael/minetest/bin/../mods/uba/test/items.conf"

and finally items.conf after execution


default:sword_diamond = 4

default:sword_wood = 4