RFR: 8367401: Parallel: Remove unused field in PSKeepAliveClosure

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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Commit messages:
 - src/hotspot/share/gc/parallel/psScavenge.cpp

Changes: https://git.openjdk.org/jdk/pull/27210/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367401
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27210.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27210/head:pull/27210

PR: https://git.openjdk.org/jdk/pull/27210