changelog: remove IXIA specific traffic generator

IXIA traffic generator is not part of the release.
The solution does not limit the traffic generator to IXIA only,
and will support any traffic generators available to end users.

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I208a76a3dfd071120e95b28c4520fb9abc04c59e