CDiskBlockIndex: remove unused ToString() class member

and mark its inherited CBlockIndex#ToString public interface member
as deleted, to disallow calling it in the derived CDiskBlockIndex class.

src/test/fuzz/chain.cpp

@@ -31,7 +31,6 @@ FUZZ_TARGET(chain)
         (void)disk_block_index->GetUndoPos();
         (void)disk_block_index->HaveTxsDownloaded();
         (void)disk_block_index->IsValid();
-        (void)disk_block_index->ToString();
     }
 
     const CBlockHeader block_header = disk_block_index->GetBlockHeader();