RFR: 8358483: G1: Remove G1HeapRegionManager::num_available_regions

[Albert Mingkun Yang]

Simple cleanup of moving an API from `G1HeapRegionManager` to `G1CollectedHeap`.

Test: tier1

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Commit messages:
 - g1-move-api

Changes: https://git.openjdk.org/jdk/pull/25611/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358483
  Stats: 12 lines in 4 files changed: 2 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25611.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25611/head:pull/25611

PR: https://git.openjdk.org/jdk/pull/25611