Revert "overcome the global.json constraint with a hack"

This reverts commit c02346c72aafd1826b0397dbbce4ce04bf252f50.

kokoro/release/csharp/windows/build_nuget.bat

@@ -2,13 +2,4 @@
 cd /d %~dp0\..\..\..\..
 
 cd csharp
-
-@rem see what dotnet version is available
-dotnet --version
-
-@rem TODO(jtattermusch): Kokoro workers currently only have dotnet SDK 2.1.3
-@rem so we just overwrite the SDK requirement in global.json as the results
-@rem should be fully compatible.
-echo { "sdk": { "version": "2.1.3" } } >global.json
-
 call build_packages.bat