RFR: 8260415: Remove unused class ReferenceProcessorMTProcMutator

[Leo Korinth]

ReferenceProcessorMTProcMutator is not used. ReferenceProcessorMTDiscoveryMutator seems to do the same and is still being used.

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Commit messages:
 - 8260415

Changes: https://git.openjdk.java.net/jdk/pull/2578/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2578&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260415
  Stats: 22 lines in 1 file changed: 0 ins; 22 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2578.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2578/head:pull/2578

PR: https://git.openjdk.java.net/jdk/pull/2578