Remove unreachable content length check for chunked encoding
_prepare_headers had a check to remove content-length
if chunked encoding was set. This code was unreachable
under normal circumstances because enabling chunked
encoding checks to see if the content-length header
is there and raises RuntimeError. It was still possible
to reach this code if RuntimeError was suppressed because
enable_chunked_encoding set self._chunked = True before
checking if it needed to raise. To make it truely
unreachable, the check is now done first in enable_chunked_encoding