[Bugfix][Benchmark] Fix --save-detailed flag being applied when not specified

[n0gu-furiosa]

In benchmark_serving.py, the --save-detailed flag is always applied, even when not specified, because the code attempts to remove detail keys from an incorrect dictionary.

This PR corrects the logic to ensure that detailed output is only saved when the flag is explicitly enabled.

[github-actions]

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[DarkLight1337]

Thanks for the bugfix! However it's no longer required after #17028 so I'll close this issue