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

[Aleksey Shipilev]

On Mon, 29 Mar 2021 17:27:36 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Shenandoah no longer uses parallel reference processor,  no reason to enable it.
> 
> Test:
> - [x] hotspot_gc_shenandoah

Looks fine and trivial.

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

Marked as reviewed by shade (Reviewer).

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