RFR: 8378348: Obsolete ParallelRefProcEnabled

[Albert Mingkun Yang]

Obsolete `ParallelRefProcEnabled`, which has `true` as its default value, for GCs (Parallel and G1) supporting this flag.

Test: tier1-5

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Commit messages:
 - obsolete-ref-proc

Changes: https://git.openjdk.org/jdk/pull/29845/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29845&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378348
  Stats: 171 lines in 10 files changed: 1 ins; 152 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/29845.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29845/head:pull/29845

PR: https://git.openjdk.org/jdk/pull/29845