RFR: 8328112: Remove CardTable::_guard_region

[Guoxiong Li]

Hi all,

This patch removes the `CardTable::_guard_region`. Thanks for taking the time to review.

Best Regards,
-- Guoxiong

-------------

Commit messages:
 - JDK-8328112

Changes: https://git.openjdk.org/jdk/pull/18324/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18324&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328112
  Stats: 12 lines in 3 files changed: 0 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18324.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18324/head:pull/18324

PR: https://git.openjdk.org/jdk/pull/18324