`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
{
    std::map<std::string, fs::path> mapBlockFiles;
    
    
    
    LogPrintf("Removing unusable blk?????.dat and rev?????.dat files for -reindex with -prune\n");
    fs::path blocksdir = gArgs.GetBlocksDirPath();
    for (fs::directory_iterator it(blocksdir); it != fs::directory_iterator(); it++) {
        if (fs::is_regular_file(*it) &&
            it->path().filename().string().length() == 12 &&
            it->path().filename().string().substr(8,4) == ".dat")
        {
            if (it->path().filename().string().substr(0,3) == "blk")
                mapBlockFiles[it->path().filename().string().substr(3,5)] = it->path();
            else if (it->path().filename().string().substr(0,3) == "rev")
                remove(it->path());
        }
    }
    
    
    
    
    int nContigCounter = 0;
    for (const std::pair<const std::string, fs::path>& item : mapBlockFiles) {
        if (atoi(item.first) == nContigCounter) {
            nContigCounter++;
            continue;