I’m having the same issue. I’ve deleted every record in the database referencing a particular field group, removed the page itself that contained the field group, removed the theme files referencing the field group. And I still have it showing up under “Tools” and I can’t re-import.
I did a database base search for the exact term and got no results. Then did a search through all of the wordpress code and got no results.
Where else could this be?