code: public ResultType deleteMaterial(String mediaId) {
          String url = BASE_API_URL + "cgi-bin/material/del_material?access_token=#";
          final Map<String, String> param = new HashMap<String, String>();
          param.put("media_id", mediaId);
          BaseResponse response = executePost(url, JSONUtil.toJson(param));
          return ResultType.get(response.getErrcode());
      }
docs: 删除一个永久素材
  @param mediaId 素材ID
  @return 删除结果