Remove dead Assistant code.

This code is never called as neither AssistantHeaderView nor
AssistantMainStage have direct children whose visibility is modified.

Bug: b:115784427
Change-Id: Ic89d0c809110d9059ee32c944c95ebcc5e21c448
Reviewed-on: https://chromium-review.googlesource.com/1241936
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593790}

ash/assistant/ui/main_stage/assistant_header_view.cc

@@ -75,10 +75,6 @@ gfx::Size AssistantHeaderView::CalculatePreferredSize() const {
   return gfx::Size(INT_MAX, GetHeightForWidth(INT_MAX));
 }
 
-void AssistantHeaderView::ChildVisibilityChanged(views::View* child) {
-  PreferredSizeChanged();
-}
-
 void AssistantHeaderView::InitLayout() {
   layout_manager_ = SetLayoutManager(std::make_unique<views::BoxLayout>(
       views::BoxLayout::Orientation::kVertical,

ash/assistant/ui/main_stage/assistant_header_view.h

@@ -31,7 +31,6 @@ class AssistantHeaderView : public views::View,
   // views::View:
   const char* GetClassName() const override;
   gfx::Size CalculatePreferredSize() const override;
-  void ChildVisibilityChanged(views::View* child) override;
 
   // AssistantInteractionModelObserver:
   void OnResponseChanged(const AssistantResponse& response) override;

ash/assistant/ui/main_stage/assistant_main_stage.cc

@@ -234,10 +234,6 @@ void AssistantMainStage::ChildPreferredSizeChanged(views::View* child) {
   PreferredSizeChanged();
 }
 
-void AssistantMainStage::ChildVisibilityChanged(views::View* child) {
-  PreferredSizeChanged();
-}
-
 void AssistantMainStage::OnViewBoundsChanged(views::View* view) {
   if (view == active_query_view_) {
     UpdateTopPadding();

ash/assistant/ui/main_stage/assistant_main_stage.h

@@ -44,7 +44,6 @@ class AssistantMainStage : public views::View,
   // views::View:
   const char* GetClassName() const override;
   void ChildPreferredSizeChanged(views::View* child) override;
-  void ChildVisibilityChanged(views::View* child) override;
 
   // views::ViewObserver:
   void OnViewBoundsChanged(views::View* view) override;