NetworkPkg/Dhcp6Dxe: Remove an unused global variable.

NetworkPkg/Dhcp6Dxe: Remove an unused global variable.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1413

The global variable mAllDhcpServersAddress has never
been used, this patch is to clean it.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Songpeng Li <songpeng.li@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>