NetworkPkg: Clean the previous address since the policy changed

NetworkPkg: Clean the previous address since the policy changed

The previous DNS server data will be retained after the policy
changes from Auto to Manual. This patch is used to clean the
previous dhcp configuration data.

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