Not sure where I saw this but is there a reason why leaving a cluster wont be simple like a few clicks? Instead of using the CLI and going through steps to remove it?
The reason is that removing a node is a somewhat dangerous operation that might incur serious data loss or downtime if executed incorrectly (e.g. the need to ensure that a removed node will never come online again while still configured), and so it is hard to make a consistent, automated way of doing it. I don't think it's too involved though, follow the instructions carefully and it's a matter of minutes to remove and re-image a node...