RFR: 8254940: AArch64: Cleanup non-product thread members

[Anton Kozlov]

Please review a very small clean up that deletes unused thread members in linux/aarch64 and windows/aarch64

Testing: grep, linux/aarch64 fastdebug build

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Commit messages:
 - Remove usused thread members

Changes: https://git.openjdk.java.net/jdk/pull/708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254940
  Stats: 19 lines in 2 files changed: 0 ins; 19 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/708.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/708/head:pull/708

PR: https://git.openjdk.java.net/jdk/pull/708