Remove file missed in GPU removal (#3551)

src/ngraph/runtime/gpu/gpu_backend_visibility.hpp

-//*****************************************************************************
-// Copyright 2017-2019 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.
-//*****************************************************************************
-
-#include "ngraph/visibility.hpp"
-
-// Now we use the generic helper definitions above to define GPU_BACKEND_API
-// GPU_BACKEND_API is used for the public API symbols. It either DLL imports or DLL exports
-//    (or does nothing for static build)
-
-#ifdef GPU_BACKEND_EXPORTS // defined if we are building the GPU_BACKEND
-#define GPU_BACKEND_API NGRAPH_HELPER_DLL_EXPORT
-#else
-#define GPU_BACKEND_API NGRAPH_HELPER_DLL_IMPORT
-#endif