RFR: 8329368: Generational ZGC: Remove the unnecessary friend classes in ZAllocator

[Guoxiong Li]

Hi all,

This trivial patch removes the unnecessary friend classes in `ZAllocator`. The tests `make test-tier1_gc` passed locally.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8329368

Changes: https://git.openjdk.org/jdk/pull/18560/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18560&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329368
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18560.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18560/head:pull/18560

PR: https://git.openjdk.org/jdk/pull/18560