traffic: rm --scale-traffic default value

traffic: rm --scale-traffic default value

-S, --scale--traffic defaults to 1.0
when we switch to new option -T, both -T and -S are set, which raise an error:

script/traffic_replay #234: --scale-traffic and --packets-per-second are incompatible. Use one or the other.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Tim Beale <timbeale@catalyst.net.nz>
Autobuild-User(master): Tim Beale <timbeale@samba.org>
Autobuild-Date(master): Wed Jan 23 03:09:41 CET 2019 on sn-devel-144