Removed obsolete flutter gallery assets (#4605)

examples/flutter_gallery/flutter.yaml

@@ -2,12 +2,6 @@ name: flutter_gallery
 uses-material-design: true
 assets:
   - assets/flutter_logo.png
-  - assets/section_animation.png
-  - assets/section_style.png
-  - assets/section_layout.png
-  - assets/section_components.png
-  - assets/section_patterns.png
-  - assets/section_usability.png
   - lib/gallery/example_code.dart
   - packages/flutter_gallery_assets/appbar_background.jpg
   - packages/flutter_gallery_assets/pesto/avatar.jpg