RFR: 8326722: Cleanup unnecessary forward declaration in collectedHeap.hpp

[Zhengyu Gu]

Trivial cleanup

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Commit messages:
 - 8326722: Cleanup unnecessary forward declaration in collectedHeap.hpp

Changes: https://git.openjdk.org/jdk/pull/18018/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326722
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18018.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18018/head:pull/18018

PR: https://git.openjdk.org/jdk/pull/18018