[BUG] stream_interface: fix retnclose and remove cond_close

[BUG] stream_interface: fix retnclose and remove cond_close

The stream_int_cond_close() function was added to preserve the
contents of the response buffer because stream_int_retnclose()
was buggy. It flushed the response instead of flushing the
request. This caused issues with pipelined redirects followed
by error messages which ate the previous response.

This might even have caused object truncation on pipelined
requests followed by an error or by a server redirection.

Now that this is fixed, simply get rid of the now useless
function.