RFR: 8260416: Remove unused method ReferenceProcessor::is_mt_processing_set_up()

[Leo Korinth]

Code is not used.

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Commit messages:
 - 8260416: Remove unused method ReferenceProcessor::is_mt_processing_set_up()

Changes: https://git.openjdk.java.net/jdk/pull/2591/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2591&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260416
  Stats: 7 lines in 2 files changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2591.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2591/head:pull/2591

PR: https://git.openjdk.java.net/jdk/pull/2591