RFR: 8293623: Simplify G1ConcurrentRefineThreadControl

[Lei Zaakjyu]

8293623: Simplify G1ConcurrentRefineThreadControl

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Commit messages:
 - simplify 'G1ConcurrentRefineThreadControl'

Changes: https://git.openjdk.org/jdk/pull/17077/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17077&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293623
  Stats: 18 lines in 2 files changed: 1 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/17077.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17077/head:pull/17077

PR: https://git.openjdk.org/jdk/pull/17077