RFR: 8261804: Remove field _processing_is_mt, calculate it instead [v2]

[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.

Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:

  Fixup suggested by Kim

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2704/files
  - new: https://git.openjdk.java.net/jdk/pull/2704/files/5b80656f..eff32142

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2704&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2704&range=00-01

  Stats: 8 lines in 2 files changed: 5 ins; 2 del; 1 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