[VL] Clean up unused variables in cpp source files

[rui-mo]

What changes were proposed in this pull request?

Clean up unused variables to avoid compilation warnings.

How was this patch tested?

With existing unit tests.

[github-actions[bot]]

Thanks for opening a pull request!

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Then could you also rename commit message and pull request title in the following format?

[GLUTEN-${ISSUES_ID}][COMPONENT]feat/fix: ${detailed message}

See also:

• Other pull requests