Delete the n'th version of this symbol from the historical collection.

def _delete_version(self, symbol, version_num, do_cleanup=True):
        """
        Delete the n'th version of this symbol from the historical collection.
        """
        version = self._versions.find_one({'symbol': symbol, 'version': version_num})
        if not version:
            logger.error("Can't delete %s:%s as not found in DB" % (symbol, version_num))
            return
        # If the version is pointed to by a snapshot, then can't delete
        if version.get('parent', None):
            for parent in version['parent']:
                snap_name = self._snapshots.find_one({'_id': parent})
                if snap_name:
                    snap_name = snap_name['name']
                logger.error("Can't delete: %s:%s as pointed to by snapshot: %s" % (symbol, version['version'],
                                                                                    snap_name))
                return
        self._versions.delete_one({'_id': version['_id']})
        # TODO: for FW pointers, if the above statement fails, they we have no way to delete the orphaned segments.
        #       This would be possible only via FSCK, or by moving the above statement at the end of this method,
        #       but with the risk of failing to delelte the version catastrophically, and ending up with a corrupted v.
        if do_cleanup:
            cleanup(self._arctic_lib, symbol, [version['_id']], self._versions,
                    shas_to_delete=tuple(bson.binary.Binary(s) for s in version.get(FW_POINTERS_REFS_KEY, [])),
                    pointers_cfgs=(get_fwptr_config(version), ))