RFR: 8369041: Release memory after testing in ThreadsRunner.java

[Albert Mingkun Yang]

Small patch of cleaning up no longer used data structures after testing is complete. 

Test: tier1-5

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Commit messages:
 - stress-free-memory

Changes: https://git.openjdk.org/jdk/pull/27606/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27606&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369041
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27606.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27606/head:pull/27606

PR: https://git.openjdk.org/jdk/pull/27606