[jdk16u] RFR: 8263030: Remove Shenandoah leftovers from ReferenceProcessor

[Aleksey Shipilev]

Shenandoah is no longer using the shared ReferenceProcessor. We can remove remaining Shenandoah leftovers there.

Additional testing:
 - [x] Linux x86_64 fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Backport ef5e13d263b6d4aaa50e777f7a3fc818293126c8

Changes: https://git.openjdk.java.net/jdk16u/pull/81/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=81&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263030
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/81.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/81/head:pull/81

PR: https://git.openjdk.java.net/jdk16u/pull/81