RFR: 8330961: Remove redundant public specifier in ModRefBarrierSet

[Albert Mingkun Yang]

Trivial removing redundant code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18911/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18911&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330961
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18911.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18911/head:pull/18911

PR: https://git.openjdk.org/jdk/pull/18911