RFR: 8280804: Parallel: Remove unused variables in PSPromotionManager::drain_stacks_depth

[Albert Mingkun Yang]

On Thu, 27 Jan 2022 14:34:40 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial change of removing dead code.
> 
> Test: build

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/7245