Strip the trailing router prompt from the output.

def strip_prompt(self, a_string):
        """Strip the trailing router prompt from the output."""
        response_list = a_string.split(self.RESPONSE_RETURN)
        new_response_list = []
        for line in response_list:
            if self.base_prompt not in line:
                new_response_list.append(line)

        output = self.RESPONSE_RETURN.join(new_response_list)
        return self.strip_context_items(output)