[flang] Fix -Wunused-variable in BoxedProcedure.cpp (NFC)

llvm-project/flang/lib/Optimizer/CodeGen/BoxedProcedure.cpp:157:12:
error: unused variable 'it' [-Werror,-Wunused-variable]
      auto it = convertedTypes.try_emplace(ty, rec);
           ^
1 error generated.