RFR: 8335706: G1: Remove G1ConcurrentRefine::RemSetSamplingClosure::_cset

[Ivan Walulya]

Trivial cleanup of unused field G1ConcurrentRefine::RemSetSamplingClosure::_cset.

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Commit messages:
 - reorder
 - Trivial

Changes: https://git.openjdk.org/jdk/pull/20035/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20035&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335706
  Stats: 5 lines in 1 file changed: 1 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20035.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20035/head:pull/20035

PR: https://git.openjdk.org/jdk/pull/20035