Remove urllib3-specific kwargs from non-urllib3 branch.

As pointed out by @lukesneeringer in #1395, the kwargs in the non-urllib3 branch of the `iter_content` code are urllib3 specific. This PR will finish the split I originally made in #1425.

/cc @sigmavirus24 and @lukesneeringer who have both done work that has been rendered unnecessary (though not useless) by this change.