Open your Applications folder in Finder and move the Little Snitch Mini app to the trash.
IMPORTANT NOTE: Do not delete the app be any other means (via Terminal, removing it from Launchpad, using third party removal tools) because this won’t uninstall the apps’s system extension.
Open the Applications folder in Finder.
Select the Little Snitch Mini application.
Move it to the trash.
You will be notified that this also removes the contained system extension.