RFR: 8264052: Shenandoah: Backout 8263832

[Zhengyu Gu]

TestStringDedupStress started to fail after JDK-8263832. 

JDK-8263832 is incorrect, because Threads::possibly_parallel_threads_do() only iterates over Java and VM threads, it should be backed out. The original works correctly.

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Commit messages:
 - JDK-8264052

Changes: https://git.openjdk.java.net/jdk/pull/3158/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3158&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264052
  Stats: 12 lines in 1 file changed: 4 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3158.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3158/head:pull/3158

PR: https://git.openjdk.java.net/jdk/pull/3158