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

[joejackson1993]

trivial cleanup

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

Commit messages:
 - 8337389: Remove unnecessary forward declarations

Changes: https://git.openjdk.org/jdk/pull/20393/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337389
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20393.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20393/head:pull/20393

PR: https://git.openjdk.org/jdk/pull/20393