Set candidate_cfg to current running-config. Erase the merge_cfg file.

def _discard_config(self):
        """Set candidate_cfg to current running-config. Erase the merge_cfg file."""
        discard_candidate = "copy running-config {}".format(
            self._gen_full_path(self.candidate_cfg)
        )
        discard_merge = "copy null: {}".format(self._gen_full_path(self.merge_cfg))
        self.device.send_command_expect(discard_candidate)
        self.device.send_command_expect(discard_merge)