remove `sort_field` option?

[rahulbot]

Right now you can sort by publication_date or indexed_date. @philbudne suggests on slack that "I'm having trouble imagining how any sort order other than indexed_date alone could work (unless there is a hidden, monotonic id)... Both _id and published_date orderings of documents are unstable: new documents can appear at any point in the ordering at any time, no?".

So should we remove the sort_field="publication_date" option since it doesn't return stable results? Someone using results for analysis can always re-sort things themselves once they have data... this is about making sure we return all matching results while paging through stories.

[pgulley]

I'm still confused as to why this only appeared with the recent change to the publication_date schema- but I'm not opposed to the removal. I feel like I would intuitively expect things to be sorted by publication_date, given that it's a part of the query- so we could even optionally re-sort in the api-client or in providers ourselves, to save potential consumer confusion