RFR: 8261842: Shenandoah: cleanup ShenandoahHeapRegionSet

[Aleksey Shipilev]

There are a couple of stale/unused methods in ShenandoahHeapRegionSet that we can eliminate instead of improving them, for example in JDK-8261838.

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

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Commit messages:
 - Update
 - 8261842: Shenandoah: cleanup ShenandoahHeapRegionSet

Changes: https://git.openjdk.java.net/jdk/pull/2592/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2592&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261842
  Stats: 88 lines in 3 files changed: 0 ins; 82 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2592.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2592/head:pull/2592

PR: https://git.openjdk.java.net/jdk/pull/2592