RFR: 8261804: Remove field _processing_is_mt, calculate it instead

[Leo Korinth]

In the reference processor, remove _processing_is_mt. Instead calculate its value in its accessor, processing_is_mt().

This change will remove some state, make RefProcMTDegreeAdjuster a bit simpler and make it much easier to derive when processing is indeed multi threaded.

-------------

Commit messages:
 - 8261804: Remove field _processing_is_mt, calculate it instead

Changes: https://git.openjdk.java.net/jdk/pull/2704/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2704&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261804
  Stats: 41 lines in 5 files changed: 2 ins; 16 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2704.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2704/head:pull/2704

PR: https://git.openjdk.java.net/jdk/pull/2704