Removes some dead code from WindowTreeClient

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

Index: components/mus/public/cpp/lib/window_tree_client_impl.h
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.h b/components/mus/public/cpp/lib/window_tree_client_impl.h
index abf688fae4da7e8aab8a5398094eebf78c3635d7..e5e11854e33f99bc174cdf93169ef02fd89ac989 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.h
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.h
@@ -190,8 +190,6 @@ class WindowTreeClientImpl : public WindowTreeConnection,
                      const mojo::String& name,
                      mojo::Array<uint8_t> transit_data) override;
 
-  mojo::Callback<void(bool)> ActionCompletedCallback();
-
   // This is set once and only once when we get OnEmbed(). It gives the unique
   // id for this connection.
   ConnectionSpecificId connection_id_;