RFR: 8265967: Unused NullCheckNode forward declaration in node.hpp

[Tobias Hartmann]

Please review this trivial change that removes the forward declaration of a nonexistent `NullCheckNode`.

Thanks,
Tobias

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Commit messages:
 - 8265967: Unused NullCheckNode forward declaration in node.hpp

Changes: https://git.openjdk.java.net/jdk/pull/3691/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3691&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265967
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3691.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3691/head:pull/3691

PR: https://git.openjdk.java.net/jdk/pull/3691