RFR: 8350667: Remove startThread_lock() and _startThread_lock on AIX

[Matthias Baesken]

The startThread_lock() and _startThread_lock coding is not used on AIX so it can be removed.

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Commit messages:
 - JDK-8350667

Changes: https://git.openjdk.org/jdk/pull/23779/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23779&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350667
  Stats: 14 lines in 3 files changed: 2 ins; 11 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23779.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23779/head:pull/23779

PR: https://git.openjdk.org/jdk/pull/23779