RFR: JDK-8328604: remove on_aix() function

[Matthias Baesken]

The on_aix() function was added to distinguish "real" AIX and os400 PASE mode. But the os400 PASE was never supported in OpenJDK and there are no plans to do so.
So we can clean this up a bit and simplify the AIX coding.

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Commit messages:
 - JDK-8328604

Changes: https://git.openjdk.org/jdk/pull/18427/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18427&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328604
  Stats: 39 lines in 2 files changed: 0 ins; 30 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18427.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18427/head:pull/18427

PR: https://git.openjdk.org/jdk/pull/18427