[jdk17] RFR: 8268350: Remove assert that ensures thread identifier remains the same

[Leo Korinth]

This fix is divided into two commits: The first commit reverts the temporary 8268537 fixup to the problem that is properly resolved in this PR, the second commit removes the assert that ensures thread identifier remains the same for JFR events. It also always reassign a new thread identifier as it can not be known that it will remain the same after first assignment.

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Commit messages:
 - 8268350: Remove assert that ensures thread identifier remains the same
 - Revert "8268537: (Temporary) Disable ParallelRefProcEnabled for Parallel GC"

Changes: https://git.openjdk.java.net/jdk17/pull/161/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=161&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268350
  Stats: 8 lines in 3 files changed: 0 ins; 5 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/161.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/161/head:pull/161

PR: https://git.openjdk.java.net/jdk17/pull/161