RFR: 8341558: [AIX] build broken after 8341413

[Martin Doerr]

The AIX build is currently broken due to a build warning caused by [JDK-8341413](https://bugs.openjdk.org/browse/JDK-8341413):
osThread_aix.cpp:43:5: error: field '_last_cpu_times' will be initialized after field '_startThread_lock' [-Werror,-Wreorder-ctor]

It turns out that `_last_cpu_times` is unused and can be removed.

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Commit messages:
 - 8341558: [AIX] build broken after 8341413

Changes: https://git.openjdk.org/jdk/pull/21369/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341558
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21369/head:pull/21369

PR: https://git.openjdk.org/jdk/pull/21369