[jdk16u] RFR: 8264374: Shenandoah: Remove leftover parallel reference processing argument

[Aleksey Shipilev]

After JDK-8254315, Shenandoah no longer uses upstream weak reference processor, it should remove related ParallelRefProcEnabled setting.

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Commit messages:
 - Backport ac604a18c92d9d21ea5b5b14fea512642d33764f

Changes: https://git.openjdk.java.net/jdk16u/pull/100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=100&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264374
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/100/head:pull/100

PR: https://git.openjdk.java.net/jdk16u/pull/100