Merge #16287: refactor: remove extra CBlockIndex declaration

9824a0d Remove extra CBlockIndex declaration (RJ Rybarczyk)

Pull request description:

  Remove duplicate `class CBlockIndex;` declaration.

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Tree-SHA512: aaf88450f53cb8859778102fe971b1121808819c04e64802e5a5cf47bf1403b42531361c52b097b41b905f9fa1bb7acc82b446cfa659c6ac41d00fab29e114e4

Author: MarcoFalke

src/validationinterface.h

@@ -16,7 +16,6 @@ extern CCriticalSection cs_main;
 class CBlock;
 class CBlockIndex;
 struct CBlockLocator;
-class CBlockIndex;
 class CConnman;
 class CValidationInterface;
 class CValidationState;