RFR: 8325436: G1: Remove unused G1RegionMarkStats::is_clear

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17751/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17751&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325436
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17751.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17751/head:pull/17751

PR: https://git.openjdk.org/jdk/pull/17751