[HttpClient] Remove dead code

Author: nicolas-grekas

Response/MockResponse.php

@@ -186,11 +186,6 @@ protected static function perform(ClientState $multi, array &$responses): void
                     $chunk[1]->getHeaders(false);
                     self::readResponse($response, $chunk[0], $chunk[1], $offset);
                     $multi->handlesActivity[$id][] = new FirstChunk();
-                    $buffer = $response->requestOptions['buffer'] ?? null;
-
-                    if ($buffer instanceof \Closure && $response->content = $buffer($response->headers) ?: null) {
-                        $response->content = \is_resource($response->content) ? $response->content : fopen('php://temp', 'w+');
-                    }
                 } catch (\Throwable $e) {
                     $multi->handlesActivity[$id][] = null;
                     $multi->handlesActivity[$id][] = $e;