Remove duplicate FENs from `position_evals`

[guidopetri]

There's ~110 duplicate FENs in position_evals.

I removed them using:

delete from position_evals
    where id in (select max(id)
                        from position_evals
                        group by fen
                        having count(*) > 1
                        order by fen);

(where the order by does not matter, of course)