Remove unnecessary consistency checks

As per https://github.com/zcash/zcash/issues/6532 this should speed up
startup times by roughly 3X and drastically reduce disk i/o on startup.

src/txdb.cpp

@@ -737,6 +737,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
 
                 //fprintf(stderr,"loadguts ht.%d\n",pindexNew->GetHeight());
                 // Consistency checks
+                /*
                 CBlockHeader header;
                 {
                     LOCK(cs_main);
@@ -753,6 +754,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
                 if (header.GetHash() != pindexNew->GetBlockHash())
                     return error("LoadBlockIndex(): block header inconsistency detected: on-disk = %s, in-memory = %s",
                                  diskindex.ToString(),  pindexNew->ToString());
+
                 if ( 0 ) // POW will be checked before any block is connected
                 {
                     uint8_t pubkey33[33];
@@ -760,6 +762,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
                     if (!CheckProofOfWork(header,pubkey33,pindexNew->GetHeight(),Params().GetConsensus()))
                         return error("LoadBlockIndex(): CheckProofOfWork failed: %s", pindexNew->ToString());
                 }
+                */
                 pcursor->Next();
             } else {
                 return error("LoadBlockIndex() : failed to read value");