Don't check if record exists in content_storage on 'insert'

It's insert, the record should not exist so no point looking for it.

Not that later on there is checking on $op based on that records should not exist on insert, e.g.

          if ($op == 'update') {
            db_query('DELETE FROM {'. $db_info['table'] .'} WHERE vid = %d', $node->vid);
          }