RFR: 8348347: Cleanup JavaThread subclass support in SA [v2]

[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.

Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:

  update comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23456/files
  - new: https://git.openjdk.org/jdk/pull/23456/files/d13d365c..da818050

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23456&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23456&range=00-01

  Stats: 5 lines in 1 file changed: 1 ins; 4 del; 0 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