RFR: 8335711: G1: Remove unused argument in G1AllocRegion

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/20038/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20038&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335711
  Stats: 9 lines in 2 files changed: 0 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20038/head:pull/20038

PR: https://git.openjdk.org/jdk/pull/20038