<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def filter_roidb(self):
 """Remove images without usable rois"""

        num_roidb = len(self._roidb)
        self._roidb = [roi_rec for roi_rec in self._roidb if len(roi_rec['gt_classes'])]
        num_after = len(self._roidb)
        logger.info('filter roidb: {} -> {}'.format(num_roidb, num_after))