RFR: 8353471: ZGC: Redundant generation id in ZGeneration

[Joel Sikström]

The ZGeneration class (and in turn ZGenerationOld and ZGenerationYoung) keeps track of its own ZGenerationId, which means that the generation id does not need to be passed along as an argument when calling internal functions.

I've removed the id parameter from `ZGeneration::select_relocation_set` in favor of using the member variable `_id`.

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Commit messages:
 - 8353471: ZGC: Redundant generation id in ZGeneration

Changes: https://git.openjdk.org/jdk/pull/24374/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24374&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353471
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/24374.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24374/head:pull/24374

PR: https://git.openjdk.org/jdk/pull/24374