MdePkg/Include/Protocol: EFI_HII POPUP_PROTOCOL duplicate declaration

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3285



Remove duplicate declaration of structure type
EFI_HII_POPUP_PROTOCOL from HiiPopup.h.  The forward
declaration of the top of this file already declared
this type.

Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>