Remove interior concrete chimney
Is there at least room for a hammer drill that would allow you to drill smallish holes in the concrete, and then use stone-splitting wedges to break it up?

This would work if it's pure masonry, but not if there's any steel rebar in the concrete.

If it's reinforced (do you see any steel ends in the cut surface in the attic?), then I can't think of any easy way to get rid of it. A diamond saw would be slow, noisy and messy. Probably time to call a professional for advice.