Simplify chunk reading

[silverjam]

A "fix" for #702 which removes the superfluous logic that built up an large chunk/upload-part sized buffer from a series of small reads. The original intent of this code was to stream small reads with small buffers up to the chunk/upload-part size. I think the original logic could be brought back with something like http_body_util::StreamBody but this at least removes a superfluous stream and the loop that builds up the buffer.