RFR: 8260263: Remove PtrQueue::_qset

[Thomas Schatzl]

Hi all,

  can I have reviews for this trivial(?) removal of dead code pertaining to and including `PtrQueue::_qset`?

Testing: local compilation, tier1

Thanks,
  Thomas

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/2194/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2194&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260263
  Stats: 25 lines in 4 files changed: 0 ins; 25 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2194.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2194/head:pull/2194

PR: https://git.openjdk.java.net/jdk/pull/2194