Remove leftovers from packages.config

src/csharp/Grpc.IntegrationTesting/packages.config

@@ -5,7 +5,6 @@
   <package id="Google.Apis.Auth" version="1.9.3" targetFramework="net45" />
   <package id="Google.Apis.Core" version="1.9.3" targetFramework="net45" />
   <package id="Google.Protobuf" version="3.0.0-alpha4" targetFramework="net45" />
-  <package id="Google.ProtocolBuffers" version="2.4.1.521" targetFramework="net45" />
   <package id="Ix-Async" version="1.2.3" targetFramework="net45" />
   <package id="Microsoft.Bcl" version="1.1.10" targetFramework="net45" />
   <package id="Microsoft.Bcl.Async" version="1.0.168" targetFramework="net45" />