RFR: 8341513: Remove the unused thread_type field from OSThread [v2]

[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)

Amit Kumar has updated the pull request incrementally with one additional commit since the last revision:

  revert back SAP's copyright header

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21532/files
  - new: https://git.openjdk.org/jdk/pull/21532/files/5bd46b0d..5c5dead5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21532&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21532&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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