RFR: 8263030: Remove Shenandoah leftovers from ReferenceProcessor

[Zhengyu Gu]

On Thu, 4 Mar 2021 14:21:50 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Shenandoah is no longer using the shared ReferenceProcessor. We can remove remaining Shenandoah leftovers there.
> 
> Testing:
>  - [x] hotspot_gc_shenandoah
>  - [x] hotspot_gc

Looks good and trivial

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

Marked as reviewed by zgu (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2829