RFR: 8329368: Generational ZGC: Remove the unnecessary friend classes in ZAllocator [v2]

[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

Guoxiong Li has updated the pull request incrementally with one additional commit since the last revision:

  Remove blank line.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18560/files
  - new: https://git.openjdk.org/jdk/pull/18560/files/5ee6e7d2..3f584542

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18560&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18560&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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