message: dnn: fix registration of custom OpenCVLayer
  do not require extensions library
diff: @@ -646,8 +646,6 @@ void InfEngineBackendNet::initPlugin(InferenceEngine::CNNNetwork& net)
  enginePtr->AddExtension(extension, 0);
  #else
  ie.AddExtension(extension, "CPU");
  - // OpenCV fallbacks as extensions.
  - ie.AddExtension(std::make_shared<InfEngineExtension>(), "CPU");
  #endif
  CV_LOG_INFO(NULL, "DNN-IE: Loaded extension plugin: " << libName);
  found = true;
  @@ -660,6 +658,10 @@ void InfEngineBackendNet::initPlugin(InferenceEngine::CNNNetwork& net)
  CV_LOG_WARNING(NULL, "DNN-IE: Can't load extension plugin (extra layers for some networks). Specify path via OPENCV_DNN_IE_EXTRA_PLUGIN_PATH parameter");
  }
  // Some of networks can work without a library of extra layers.
  +#if INF_ENGINE_VER_MAJOR_GT(INF_ENGINE_RELEASE_2019R1)
  + // OpenCV fallbacks as extensions.
  + ie.AddExtension(std::make_shared<InfEngineExtension>(), "CPU");
  +#endif
  #ifndef _WIN32
  // Limit the number of CPU threads.
  #if INF_ENGINE_VER_MAJOR_LE(INF_ENGINE_RELEASE_2019R1)
  