remove unused namespaces from example classes

src/csharp/Grpc.Examples.MathClient/MathClient.cs

@@ -13,9 +13,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 #endregion
-using System;
-using System.Runtime.InteropServices;
-using System.Threading;
 using System.Threading.Tasks;
 using Grpc.Core;
 

src/csharp/Grpc.Examples.MathServer/MathServer.cs

@@ -15,8 +15,6 @@
 #endregion
 
 using System;
-using System.Runtime.InteropServices;
-using System.Threading;
 using System.Threading.Tasks;
 using Grpc.Core;
 

src/csharp/Grpc.Examples/MathServiceImpl.cs

@@ -16,9 +16,7 @@
 
 #endregion
 
-using System;
 using System.Collections.Generic;
-using System.Threading;
 using System.Threading.Tasks;
 using Grpc.Core;
 using Grpc.Core.Utils;