feat(piecesScheduledRemove): Remove Piece Metadata

[wjmelements]

Reviewer @rvagg
This reduces storage accumulation.
However the piece metadata will still not be removed when the whole data set is deleted.

Changes

• rm piece metadata in piecesScheduledRemove

[rvagg]

There's a missing window here though, piece won't actually be removed from data set until the nextProvingPeriod is called on that data set. So now we have a piece with no metadata - user, or others, will list pieces in a data set, see them, but with nothing about them.

Not a great state of affairs. Would it be practical to extend PDPListener#nextProvingPeriod to pass through uint256[] removals? I wonder if that could get too big to pass as an argument there...