Remove checkpoints

The headers presync logic should be enough to prevent memory DoS using
low-work headers. Therefore, we no longer have any use for checkpoints.

src/net_processing.cpp

@@ -1789,7 +1789,6 @@ void PeerManagerImpl::MaybePunishNodeForBlock(NodeId nodeid, const BlockValidati
             break;
         }
     case BlockValidationResult::BLOCK_INVALID_HEADER:
-    case BlockValidationResult::BLOCK_CHECKPOINT:
     case BlockValidationResult::BLOCK_INVALID_PREV:
         if (peer) Misbehaving(*peer, message);
         return;