RFR: 8280419: Remove dead code related to VerifyThread and verify_thread()

[David Holmes]

This is code that was copied from the sparc port into the arm, ppc and s390 ports, but was never actually implemented.

I can't really test those platforms, but hopefully GHA builds will suffice, otherwise I need the various porters to confirm the changes.

Thanks.

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Commit messages:
 - 8280419: Remove dead code related to VerifyThread and verify_thread()

Changes: https://git.openjdk.org/jdk/pull/12506/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12506&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280419
  Stats: 80 lines in 18 files changed: 0 ins; 62 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/12506.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12506/head:pull/12506

PR: https://git.openjdk.org/jdk/pull/12506