RFR: 8341513: Remove the unused thread_type field from OSThread

[Amit Kumar]

This trivial PR cleans up ` thread_type` field. 

Build is fine on s390x-linux. I have tested on local system Mac-M1 but it failed due to this issue: [JDK-8340341](https://bugs.openjdk.org/browse/JDK-8340341)

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Commit messages:
 - updates copyright headers
 - cleanup _thread_type

Changes: https://git.openjdk.org/jdk/pull/21532/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341513
  Stats: 44 lines in 9 files changed: 0 ins; 36 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/21532.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21532/head:pull/21532

PR: https://git.openjdk.org/jdk/pull/21532