autoremove: Return when not removing

[Logicer16]

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Checks if the removable formulae after removing cask dependancies, preventing output like this:

$ brew autoremove
==> Uninstalling 0 unneeded formulae:

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @Logicer16!