Fix compile warning in nested_json_gpu.cu

[davidwendt]

Description

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.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-22.10@096bbc4). Click here to learn what that means.
The diff coverage is n/a.

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[bdice]

@gpucibot merge