NetworkPkg: Remove ping6 and ifconfig shell application.

NetworkPkg: Remove ping6 and ifconfig shell application.

Edk2 has duplicated ping6/ifconfig6 implementation in NetworkPkg and ShellPkg.
The usage and parameter format of these 2 versions are exactly same. These two
commands have been added to Shell specification so the copy under
  ShellPkg\Library\UefiShellNetwork2CommandsLib\
will be actively maintained in future.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>