RFR: JDK-8327210: AIX: Delete obsolete parameter Use64KPagesThreshold

[Joachim Kern]

As the configuration parameter Use64KPagesThreshold on AIX is not needed anymore, we remove it from the code.

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Commit messages:
 - JDK-8327210

Changes: https://git.openjdk.org/jdk/pull/18171/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18171&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327210
  Stats: 18 lines in 3 files changed: 0 ins; 14 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18171.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18171/head:pull/18171

PR: https://git.openjdk.org/jdk/pull/18171