remove code gen bat script in dotnet cli example

examples/csharp/helloworld-from-cli/generate_protos.bat

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-
-@rem Generate the C# code for .proto files
-
-setlocal
-
-@rem enter this directory
-cd /d %~dp0
-
-set TOOLS_PATH=packages\Grpc.Tools.1.0.0\tools\windows_x86
-
-%TOOLS_PATH%\protoc.exe -I../../protos --csharp_out Greeter  ../../protos/helloworld.proto --grpc_out Greeter --plugin=protoc-gen-grpc=%TOOLS_PATH%\grpc_csharp_plugin.exe
-
-endlocal