code: public function del($file)
      {
          $key         = trim($file);
          $url         = "{$this->QINIU_RS_HOST}/delete/" . self::qiniuEncode("{$this->bucket}:{$key}");
          $accessToken = $this->accessToken($url);
          $response    = $this->request($url, 'POST', ['Authorization' => "QBox $accessToken"]);
          return $response;
      }
docs: 删除单个文件