RFR: 8325186: JVMTI VirtualThreadGetThreadStateClosure class is no longer used and should be removed

[Koichi Sakata]

This pull request removes an unused class. 

I have investigated when this class was no longer in use. In the initial push of the virtual thread code (21972f45a64741ec3002499ef532e363cc3e9f93), there was only the definition of this class. I also checked subsequent commits, and it has never been used since the push.

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Commit messages:
 - Remove JVMTI VirtualThreadGetThreadStateClosure class

Changes: https://git.openjdk.org/jdk/pull/18341/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18341&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325186
  Stats: 42 lines in 2 files changed: 0 ins; 42 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18341.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18341/head:pull/18341

PR: https://git.openjdk.org/jdk/pull/18341