RFR: 8348347: Cleanup JavaThread subclass support in SA

[Chris Plummer]

Cleanup SA JavaThread support. Details in first comment:

Testing:
- Tier1
- Tier2 svc
- Tier3
- Tier5 svc
- Local testing of debuggee using graal java compiler threads. Verified that the compiler threads shows up in jstack output if the property is set.

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Commit messages:
 - fix whitespace
 - Cleanup JavaThread support

Changes: https://git.openjdk.org/jdk/pull/23456/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348347
  Stats: 429 lines in 16 files changed: 61 ins; 330 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/23456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23456/head:pull/23456

PR: https://git.openjdk.org/jdk/pull/23456