The removal of the _xmlsignatures sub-storage in
DocumentSignatureHelper::OpenSignatureStream() serves 3 purposes:

1) Remove no longer needed signature streams
2) Truncate signatures relation stream
3) Truncate still needed signature streams

2) and 3) could be done using io::XTruncate as well, but if the whole
storage is removed to handle 1), then individual truncate() calls are
not necessary.

Change-Id: Id9ed9c87c94f340dc947124b28f085561798d361