Short Answer
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.
Long Answer
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Open the Applications folder in Finder.
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Select the Little Snitch Mini application.
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Move it to the trash.
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You will be notified that this also removes the contained system extension.