Remove "when" from OData-specific items in delta responses

We added when to added/deleted links in ATom because it was part of the Atom tombstone specification for deleted entries. We added when to JSON to be consistent with Atom.

However, because we impose an ordering on the returned changes that leads to eventual consistency, knowing "when" a change occurred is of dubious value.

As we don't currently have an annotation for last change in JSON, nor do we have a way for the client to say whether or not they want/care about such information, we should remove the "when" name/value pair from added/deleted links and tombstones in JSON, so it only appears for well-formedness in Atom tombstones with no semantic meaning in OData, and perhaps we can get it optional there as well.