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

[Zhengyu Gu]

On Sat, 5 Apr 2025 00:27:05 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Please review this trivial cleanup to remove unused forward declarations.

Thanks, @kimbarrett

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PR Comment: https://git.openjdk.org/jdk/pull/24464#issuecomment-2781082245