RFR: 8370234: Remove CardTableBarrierSet::write_region

[Albert Mingkun Yang]

Trivial removing effectively dead code.

Test: tier1

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Commit messages:
 - g1-barrier-set-remove

Changes: https://git.openjdk.org/jdk/pull/27898/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27898&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370234
  Stats: 13 lines in 5 files changed: 0 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27898.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27898/head:pull/27898

PR: https://git.openjdk.org/jdk/pull/27898