validation: Load pindexBestHeader in ChainMan

Now BlockManager::LoadBlockIndex() will ACTUALLY only load BlockMan
members.

[META] In a later commit, pindexBestHeader will be moved to ChainMan as
       a member

-----

Code Reviewer Notes

Call graph of relevant functions:

ChainstateManager::LoadBlockIndex() <-- Moved to
    calls BlockManager::LoadBlockIndexDB()
        which calls BlockManager::LoadBlockIndex() <-- Moved from

There is only one call to each of inner functions, meaning that no
behavior is changing.

src/node/blockstorage.cpp

@@ -285,8 +285,6 @@ bool BlockManager::LoadBlockIndex(const Consensus::Params& consensus_params)
         if (pindex->pprev) {
             pindex->BuildSkip();
         }
-        if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == nullptr || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
-            pindexBestHeader = pindex;
     }
 
     return true;