RFR: 8330733: Generational ZGC: Remove ZBarrier::verify_old_object_live_slow_path

[Guoxiong Li]

Hi all,

This trivial patch removes the unused method `ZBarrier::verify_old_object_live_slow_path`.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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

Commit messages:
 - Remove unused method.

Changes: https://git.openjdk.org/jdk/pull/18872/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330733
  Stats: 14 lines in 2 files changed: 0 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18872/head:pull/18872

PR: https://git.openjdk.org/jdk/pull/18872