Fix {stream, {self, once}} in httpc

Previously the only difference between {stream, self} and {stream,
{self, once}} was an extra Pid in the stream_start message due to a
bug in httpc_handler. It was immediately sending a bunch of messages
till the end instead of waiting for httpc:stream_next/1 being called.
Remove http_chunk:decode/4 as it is no longer used

http_chunk:decode/4, the side-effecting chunk decoder, was only used
by httpc_handler:handle_http_body/2 but now it's completely unused.