RFR: 8269303: Remove unnecessary forward declaration of PSPromotionManager in cpCache.hpp

[Zhengyu Gu]

Please review this trivial change to remove the unnecessary forward declaration.

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Commit messages:
 - 8269303: Remove unnecessary forward declaration of PSPromotionManager in cpCache.hpp

Changes: https://git.openjdk.java.net/jdk/pull/4585/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4585&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269303
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4585.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4585/head:pull/4585

PR: https://git.openjdk.java.net/jdk/pull/4585