Fix compile warning in nested_json_gpu.cu (#11607)

Fixes compile warning introduced in #11534 
```
/cudf/cpp/src/io/json/nested_json_gpu.cu(970): warning #177-D: variable "single_item_count" was declared but never referenced
```
Removed unreferenced variable declaration.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Bradley Dice (https://github.com/bdice)

URL: #11607

cpp/src/io/json/nested_json_gpu.cu

@@ -967,7 +967,6 @@ std::pair<rmm::device_uvector<PdaTokenT>, rmm::device_uvector<SymbolOffsetT>> ge
   rmm::cuda_stream_view stream,
   rmm::mr::device_memory_resource* mr)
 {
-  constexpr std::size_t single_item_count = 1ULL;
   rmm::device_uvector<PdaTokenT> tokens{json_in.size(), stream, mr};
   rmm::device_uvector<SymbolOffsetT> tokens_indices{json_in.size(), stream, mr};
   rmm::device_scalar<SymbolOffsetT> num_written_tokens{stream, mr};