NetworkPkg/Ip6Dxe: Ignore duplicated DNS address check

Having duplicated DNS server IPs specified is not an ideal
configuration, but not an error condition. This patch is to
remove the duplicated DNS address check to allow the same DNS
address setting in SetData().

Cc: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Cc: Subramanian Sriram <sriram-s@hpe.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Subramanian Sriram <sriram-s@hpe.com>
Reviewed-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Tested-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>