RFR: 8268964: Unused ReferenceProcessorAtomicMutator

[Albert Mingkun Yang]

Simple change of removing an unused class and a method solely used by it.

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Commit messages:
 - atomic-discovery

Changes: https://git.openjdk.java.net/jdk/pull/4519/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4519&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268964
  Stats: 22 lines in 1 file changed: 0 ins; 22 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4519.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4519/head:pull/4519

PR: https://git.openjdk.java.net/jdk/pull/4519