You should be able to split the remains with a very small cold chisel. Once
you have started a split, you can pick at it with a small pin punch.
Alternating between the two ought to do it. But it needs care and patience. Failing
that a dentist's drill or a Dremel. I have picked out more delicate components
than cotter pins with no more than the lightest mark on the metal beneath
and usually no witness mark at all.
Stuart Tallack, Felpham, Great Britain