Removing pessimizing moves.

These have been reported by a (very recent) build of clang and its new -Wpessimizing-move option.
diff --git a/test/cpp/end2end/mock_test.cc b/test/cpp/end2end/mock_test.cc
index 4450e29..9c35fed 100644
--- a/test/cpp/end2end/mock_test.cc
+++ b/test/cpp/end2end/mock_test.cc
@@ -245,7 +245,7 @@
   void ResetStub() {
     std::shared_ptr<Channel> channel =
         CreateChannel(server_address_.str(), InsecureCredentials());
-    stub_ = std::move(grpc::cpp::test::util::TestService::NewStub(channel));
+    stub_ = grpc::cpp::test::util::TestService::NewStub(channel);
   }
 
   std::unique_ptr<grpc::cpp::test::util::TestService::Stub> stub_;