[flang] Remove unused variable 'nullBoxTy' in ConvertExpr.cpp (NFC)

/data/llvm-project/flang/lib/Lower/ConvertExpr.cpp:728:10: error: unused variable 'nullBoxTy' [-Werror,-Wunused-variable]
    auto nullBoxTy = builder.getRefType(fir::BoxType::get(noneTy));
         ^
1 error generated.