RFR: 8264374: Shenandoah: Remove leftover parallel reference processing argument

[Zhengyu Gu]

Shenandoah no longer uses parallel reference processor,  no reason to enable it.

Test:
- [x] hotspot_gc_shenandoah

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

Commit messages:
 - JDK-8264374

Changes: https://git.openjdk.java.net/jdk/pull/3246/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3246&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/jdk/pull/3246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3246/head:pull/3246

PR: https://git.openjdk.java.net/jdk/pull/3246