RFR: 8372163: G1: Remove unused G1HeapRegion::remove_code_root

[Albert Mingkun Yang]

Trivial removing dead code.

Test: build

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Commit messages:
 - g1-trivial-remove

Changes: https://git.openjdk.org/jdk/pull/28392/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28392&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372163
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28392.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28392/head:pull/28392

PR: https://git.openjdk.org/jdk/pull/28392