Platform/PurleyOpenBoardPkg: Remove --nt32 flag of edkseup

Since Nt32Pkg has been removed from edk2 repo, and
we will also remove the --nt32 flag in edksetup.bat.
So here update platform to pass build.

Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>