RFR: 8317535 Shenandoah: Remove unused code

[William Kemper]

Tested with `hotspot_gc_shenandoah`, `specjbb`, `specjvm`, `dacapo`, `extremem` and `heapothesys`.

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Commit messages:
 - Merge upstream
 - Remove unused code and other minor fixes

Changes: https://git.openjdk.org/jdk/pull/16104/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16104&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317535
  Stats: 185 lines in 20 files changed: 0 ins; 179 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16104.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16104/head:pull/16104

PR: https://git.openjdk.org/jdk/pull/16104