remove unused DCGCalculator::CalDCGAtK()

[jameslamb]

cppcheck shows the following warning (#4539 (comment))

src/metric/dcg_calculator.cpp:110:0: style: The function 'CalDCGAtK' is never used. [unusedFunction]

I confirmed that this method is not called anywhere in the library, using git grep CalDCGAtK. I think it's safe to remove, since only the code in src/c_api.cpp is considered parrt of the public API for LightGBM.

I also double-checked and don't see this method referenced in the in-progress CUDA work (#4528, #4630).

Notes for Reviewers

I checked the git blame to see if this method had been added for a specific purpose in the past, but don't see anything like that. It was part of the first commit on the project.

https://github.com/microsoft/LightGBM/blame/e95d5ab849f046b8fdf755441e5767094aeb8de2/src/metric/dcg_calculator.cpp#L56

[shiyu1994]

@jameslamb Thanks for the careful check. Since we are now evaluating NDCGs at multiple positions at once, only CalDCG is found to be used, instead of CalDCGAtK. I agree that it is safe to remove this.

[shiyu1994]

Approve given all the checks have been passed.

[github-actions]

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