When we check the availability of the pieces, we do:
```
result.NumUnhealthyRetrievable = len(result.ClumpedPiecesSet) + len(result.OutOfPlacementPiecesSet)
// + some magic if there are overlaps between them
numHealthy := len(pieces) - len(piecesCheck.MissingPiecesSet) - piecesCheck.NumUnhealthyRetrievable
```
This works only if OutOfPlacementPieceSet doesn't contain the offline nodes (which are already included in MissingPieceSet).
But `result.OutOfPlacementPieces.Set` should include all the nodes (even offline), as in case of lucky conditions, we are able to remove those pieces from DB.
The solution is to remove all offline nodes from `NumUnhealthyRetrievable`.
Change-Id: I90baa0396352dd040e1e1516314b3271f8712034
