RFR: 8337389: Parallel: Remove unnecessary forward declarations in psScavenge.hpp

[joejackson1993]

On Tue, 30 Jul 2024 18:14:36 GMT, joejackson1993 <duke at openjdk.org> wrote:

> trivial cleanup

still waiting on oca confirmation.

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PR Comment: https://git.openjdk.org/jdk/pull/20393#issuecomment-2318817050