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

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/7245/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7245&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280804
  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7245.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7245/head:pull/7245

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