Just move the Little Snitch application in Finder from your Applications folder to the trash.
This will completely remove all components of Little Snitch, including all its system extensions and helper tools.
IMPORTANT: Do not remove the Little Snitch app by any other means (like Terminal or some third party app-removal tool) because otherwise macOS won’t remove the Little Snitch system extension!
Your configuration data (which is stored at /Library/Application Support/Objective Development/Little Snitch) will not be deleted. So if you decide to reinstall Little Snitch at a later point, your rules and preferences will still be in place.