Remove sort order from post params. 

We don't allow that anymore since sort order has moved to the sheet theme.
@mjuhasz @DavidHuie 
@mjuhasz The sort order returned as part of the list resource is simply being remapped to use the list theme. POSTing to a list using the sort order param does not do anything since this parameter no longer exists on custom lists (this has been the case since March when we launched the new sheets).

The original plan was to remove it from the API altogether but we wanted to make sure no one was relying on that parameter in client applications. At the time, no one was, but since it's been a while, I wanted to confirm before removing it entirely.