`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.
{
    for (std::set<int>::iterator it = setFilesToPrune.begin(); it != setFilesToPrune.end(); ++it) {
        FlatFilePos pos(*it, 0);
        fs::remove(BlockFileSeq().FileName(pos));
        fs::remove(UndoFileSeq().FileName(pos));
        LogPrintf("Prune: %s deleted blk/rev (%05u)\n", __func__, *it);
    }
}
void BlockManager::FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPruneHeight, int chain_tip_height)