RFR: 8353753: Remove unnecessary forward declaration in oop.hpp

[Zhengyu Gu]

Please review this trivial cleanup to remove unused forward declarations.

-------------

Commit messages:
 - 8353753: Remove unnecessary forward declaration in oop.hpp

Changes: https://git.openjdk.org/jdk/pull/24464/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24464&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353753
  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24464.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24464/head:pull/24464

PR: https://git.openjdk.org/jdk/pull/24464