fix C# nuget restore

src/csharp/Grpc.IntegrationTesting.QpsWorker/packages.config

@@ -1,3 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<packages>
-</packages>

src/csharp/Grpc.IntegrationTesting.StressClient/packages.config

@@ -1,3 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<packages>
-</packages>

tools/run_tests/helper_scripts/pre_build_csharp.bat

@@ -89,18 +89,6 @@ if exist %NUGET% (
   %NUGET% restore -PackagesDirectory ../packages || goto :error
   cd ..
 
-  cd Grpc.IntegrationTesting.Client || goto :error
-  %NUGET% restore -PackagesDirectory ../packages || goto :error
-  cd ..
-
-  cd Grpc.IntegrationTesting.QpsWorker || goto :error
-  %NUGET% restore -PackagesDirectory ../packages || goto :error
-  cd ..
-
-  cd Grpc.IntegrationTesting.StressClient || goto :error
-  %NUGET% restore -PackagesDirectory ../packages || goto :error
-  cd ..
-
   cd Grpc.IntegrationTesting || goto :error
   %NUGET% restore -PackagesDirectory ../packages || goto :error
 

tools/run_tests/helper_scripts/pre_build_csharp.sh

@@ -73,18 +73,6 @@ then
   nuget restore -PackagesDirectory ../packages
   cd ..
 
-  cd Grpc.IntegrationTesting.Client
-  nuget restore -PackagesDirectory ../packages
-  cd ..
-
-  cd Grpc.IntegrationTesting.QpsWorker
-  nuget restore -PackagesDirectory ../packages
-  cd ..
-
-  cd Grpc.IntegrationTesting.StressClient
-  nuget restore -PackagesDirectory ../packages
-  cd ..
-
   cd Grpc.IntegrationTesting
   nuget restore -PackagesDirectory ../packages
   cd ..