RFR: 8351651: Cleanup LockStack::is_owning_thread

[Coleen Phillimore]

This is a minor mostly trivial change I found while reading through code looking for something else.
Tested with tier1-4.

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Commit messages:
 - 8351651: Cleanup LockStack::is_owning_thread

Changes: https://git.openjdk.org/jdk/pull/23981/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23981&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351651
  Stats: 27 lines in 4 files changed: 8 ins; 15 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23981.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23981/head:pull/23981

PR: https://git.openjdk.org/jdk/pull/23981