RFR: 8253392: remove PhaseCCP_DCE declaration

[Xin Liu]

hello, reviewers,
May I ask to review this trivial patch?
it's a clean-up. The forward declaration of PhaseCCP_DCE is not useful.

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Commit messages:
 - 8253392: remove PhaseCCP_DCE declaration

Changes: https://git.openjdk.java.net/jdk/pull/277/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=277&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253392
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/277.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/277/head:pull/277

PR: https://git.openjdk.java.net/jdk/pull/277