RFR: 8368213: ZGC: Cleanup dead code, unimplemented declarations, unused private fields

[Axel Boldt-Christmas]

Over the last years refactoring and bug fixes a bit of dead code has slipped through. Clean up some of this. Purposely do not touch unused public declarations with an implementation (most if not all are trivial property checks or simple constructions which have negligible maintenance cost to keep.) nor unused public typedefs.

The `is_high_usage` helper was rewritten to take a boolean rather than an optional templated print function pointer, as there was only one usage (a lambda which captured nothing).

Also updated `GCThreadLocalData` size to reflect the changes that has occurred over time. Maybe we can derive this size from all the implementors in the future, but may be to hard to do in a non hack-y way.

* Testing
  * Running ZGC test tier 1-5 on Oracle supported platforms.

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Commit messages:
 - 8368213: ZGC: Cleanup dead code, unimplemented declarations, unused private fields

Changes: https://git.openjdk.org/jdk/pull/27415/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368213
  Stats: 46 lines in 9 files changed: 1 ins; 36 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/27415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27415/head:pull/27415

PR: https://git.openjdk.org/jdk/pull/27415