RFR: 8264372: Threads::destroy_vm only ever returns true

[David Holmes]

Trivial cleanup to remove the bool return from Threads::destroy_vm as it only ever returned true.

Testing: tiers 1-3

Thanks,
David

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Commit messages:
 - 8264372: Threads::destroy_vm only ever returns true

Changes: https://git.openjdk.java.net/jdk/pull/3535/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3535&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264372
  Stats: 15 lines in 3 files changed: 0 ins; 9 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3535.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3535/head:pull/3535

PR: https://git.openjdk.java.net/jdk/pull/3535