Removed map_loader.hpp

- Map loading function is directly taken cared in occ_grid_loader.hpp file

nav2_map_server/include/nav2_map_server/map_loader.hpp

-// Copyright (c) 2018 Intel Corporation
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef NAV2_MAP_SERVER__MAP_LOADER_HPP_
-#define NAV2_MAP_SERVER__MAP_LOADER_HPP_
-
-#include <string>
-
-namespace nav2_map_server
-{
-
-class MapLoader
-{
-public:
-  virtual ~MapLoader() {}
-
-  virtual void loadMapFromFile(const std::string & filename) = 0;
-  virtual void startServices() = 0;
-};
-
-}  // namespace nav2_map_server
-
-#endif  // NAV2_MAP_SERVER__MAP_LOADER_HPP_