Update results strategy after contexture-elasticsearch goes live

[doug-patterson]

Right now we've instituted special handling for the _source property on results objects in the results strategy. This property of results will be going away with futil-js/contexture-elasticsearch#149, so when that's merged we can remove the then-unnecessary handling of _source from https://github.com/smartprocure/contexture-export/blob/master/src/dataStrategies.js#L55-L65