RFR: JDK-8269110: ZGC: Remove dead code in zBarrier

[Hamlin Li]

this is a trivial cleanup to remove some dead code in zBarrier.

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Commit messages:
 - remove dead code in zBarrier

Changes: https://git.openjdk.java.net/jdk/pull/4649/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4649&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269110
  Stats: 13 lines in 2 files changed: 0 ins; 13 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4649.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4649/head:pull/4649

PR: https://git.openjdk.java.net/jdk/pull/4649