Remove duplicate DAD entry in IP6 driver to fix DAD fail issue.

Remove duplicate DAD entry in IP6 driver to fix DAD fail issue.

The IP6 driver may create duplicate IP6_DAD_ENTRY in DupAddrDetectList in some situation like:
1. Address policy switch but not clear the delay node list, OR
2. Set manual address repeatedly before the previous DAD is finished.
The NS sent out by duplicate DAD entry will mix up with the loop back multicast packet, result in DAD fail.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17263 6f19259b-4bc3-4df7-8a09-765794883524