Removes some dead code from WindowTreeClient

components/mus/public/cpp/lib/window_tree_client_impl.cc

Index: components/mus/public/cpp/lib/window_tree_client_impl.cc
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.cc b/components/mus/public/cpp/lib/window_tree_client_impl.cc
index b4dfc50e4207161806eff6a36bca84a4605dba12..c503669ac698f6f903db305efd7cd35377fad2c2 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.cc
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.cc
@@ -728,11 +728,4 @@ void WindowTreeClientImpl::WmSetProperty(uint32_t change_id,
   tree_->WmResponse(change_id, result);
 }
 
-////////////////////////////////////////////////////////////////////////////////
-// WindowTreeClientImpl, private:
-
-mojo::Callback<void(bool)> WindowTreeClientImpl::ActionCompletedCallback() {
-  return [this](bool success) {};
-}
-
 }  // namespace mus