RFR: 8324582: Replace -Djava.util.concurrent.ForkJoinPool.common.parallelism to -Djdk.virtualThreadScheduler.parallelism in jvmti vthread tests

[Leonid Mesnik]

Some jvmti tests use
-Djava.util.concurrent.ForkJoinPool.common.parallelism
to control the pool of virtual threads. However, it is controlled by
jdk.virtualThreadScheduler.parallelism property.

The non-continuations implementation doesn't use any of these properties and it was just deleted.

I verified the fix using jcmd Thread.dump and ran all jvmti tests in the default configuration.

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Commit messages:
 - 8324582: Replace -Djava.util.concurrent.ForkJoinPool.common.parallelism to -Djdk.virtualThreadScheduler.parallelism in jvmti vthread tests

Changes: https://git.openjdk.org/jdk/pull/17547/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17547&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324582
  Stats: 15 lines in 6 files changed: 0 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/17547.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17547/head:pull/17547

PR: https://git.openjdk.org/jdk/pull/17547