docs: remove support for IPv6 in dpdk guide

Remove IPv6 related description to be consistent between DPDK and VPP.
Full support for IPv6 feature will be added in later release.

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: Ib6a5d578c3140e1458ae5eac7f15d20abd7e4e95