Removes some dead code from WindowTreeClient

No one uses this anymore.

BUG=none
TEST=none
R=sadrul@chromium.org

Review URL: https://codereview.chromium.org/1510413004

Cr-Commit-Position: refs/heads/master@{#364492}

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

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_;