reset last-returned element in EnumerationIterator.remove

[rootvector2]

remove() never nulls last, so a second call without an intervening next() deletes another element equal to the last returned one from the backing collection instead of throwing IllegalStateException; found sweeping the package's remove() methods, where the sibling adapters (EntrySetMapIterator, ZippingIterator, IteratorChain) already clear their last-returned field.

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