traffic: uniform stats output

traffic: uniform stats output

The original code is trying to output different data format for tty or file.
This is unnecessary and cause confusion while writing script to parse result.

The human-readable one is also easy for code to parse.
Remove if check for isatty(), just make output the same.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>