Remove unused conversation response code

- Remove get_responses()
- Remove get_response_button_text()
- Remove title key from $conv_responses array
- Address https://github.com/friendica/friendica/issues/8000#issuecomment-589263329

src/Object/Thread.php

@@ -34,6 +34,7 @@ use Friendica\Util\Security;
  */
 class Thread
 {
+	/** @var Post[] */
 	private $parents = [];
 	private $mode = null;
 	private $writable = false;