Remove sort order from post params. 

We don't allow that anymore since sort order has moved to the sheet theme.
@mjuhasz @DavidHuie 
* The line below the one you removed should also be removed.
* the [public documentation page](http://nationbuilder.com/lists_api) will have to be regenerated (anybody from the web services team can do that) - updating on GitHub is not enough
* removing fields from the REST resource is a breaking change, clients may expect it to be there. Is it still present in the index endpoint json, etc? I see `sort_order` in several examples on this page.