refactoring: make pindexBestInvalid internal to validation.cpp

There's no need to have this member live on CChainState since it's only used
in validation.cpp.

src/validation.h

@@ -561,7 +561,6 @@ public:
     //! The current chain of blockheaders we consult and build on.
     //! @see CChain, CBlockIndex.
     CChain m_chain;
-    CBlockIndex *pindexBestInvalid = nullptr;
     /**
      * The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS (for itself and all ancestors) and
      * as good as our current tip or better. Entries may be failed, though, and pruning nodes may be