Because removing the file is what we're trying to do. If the file was already removed, then someone did our work for us. But that's not a reason to return an error.

In a straight-up `rm()` method that removes only the listed path, returning `FileNotFound` makes sense. We wanted to perform a single action, and we couldn't perform that action.

But in `rmdir_recursive()`, we're trying to perform all actions necessary to delete a given directory. If we determine that we need to delete a file, and then discover the file was already deleted, returning an error doesn't make sense because we were never told to explicitly delete that file. That was something we told ourselves to do, in order to fulfill the desired action. Since we told ourselves to do it, we also handle the error ourselves.