refactor: remove unused forward declarations in validationinterface.h

Author: jonatack

src/validationinterface.h

@@ -17,9 +17,7 @@ class BlockValidationState;
 class CBlock;
 class CBlockIndex;
 struct CBlockLocator;
-class CConnman;
 class CValidationInterface;
-class uint256;
 class CScheduler;
 enum class MemPoolRemovalReason;