Remove urllib3-specific kwargs from non-urllib3 branch.
diff --git a/requests/models.py b/requests/models.py
index 6600a91a0f..3672b3774d 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -544,7 +544,7 @@ def generate():
             except AttributeError:
                 # Standard file-like object.
                 while 1:
-                    chunk = self.raw.read(chunk_size, decode_content=True)
+                    chunk = self.raw.read(chunk_size)
                     if not chunk:
                         break
                     yield chunk

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 This PR will finish the split I originally made in #1425.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.
