RFR: 8377161: G1: Remove unnecessary member G1FullGCScope::_g1h

[Thomas Schatzl]

Hi all,

  please review this trivial change that inlines the `_g1h` member use in ´G1FullGCScope` to be allowed to remove it.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8377161

Changes: https://git.openjdk.org/jdk/pull/29566/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29566&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377161
  Stats: 5 lines in 2 files changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29566.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29566/head:pull/29566

PR: https://git.openjdk.org/jdk/pull/29566