RFR: 8337641: G1: Remove unused G1CollectedHeap::alloc_highest_free_region

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - g1-trivial

Changes: https://git.openjdk.org/jdk/pull/20415/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337641
  Stats: 18 lines in 2 files changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20415/head:pull/20415

PR: https://git.openjdk.org/jdk/pull/20415