RFR: 8349571: Remove JavaThreadFactory interface from SA

[Chris Plummer]

The SA JavaThreadFactory interface is no longer used, so it is being removed. I stumbled on it while looking at JavaThread related code. It looks like it's original purpose is related to JavaThread subclasses at one point being platform dependent, as described in the following JavaThread.java comment:


/** This is an abstract class because there are certain OS- and
    CPU-specific operations (like the setting and getting of the last
    Java frame pointer) which need to be factored out. These
    operations are implemented by, for example,
    SolarisSPARCJavaThread, and the concrete subclasses are
    instantiated by the JavaThreadFactory in the Threads class. */


However, this is no longer the case, and this comment is being removed as part of [JDK-8348347](https://bugs.openjdk.org/browse/JDK-8348347).

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Commit messages:
 - Remove JavaThreadFactory

Changes: https://git.openjdk.org/jdk/pull/23505/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23505&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349571
  Stats: 40 lines in 2 files changed: 0 ins; 40 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23505.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23505/head:pull/23505

PR: https://git.openjdk.org/jdk/pull/23505