remove some images

remove some images from app/assets/images/active_admin which has no reference in the code base:
- [app/assets/images/active_admin/admin_notes_icon.png](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/admin_notes_icon.png) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=admin_notes_icon)]
- [app/assets/images/active_admin/index_list_icons/block_icon.svg](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/index_list_icons/block_icon.svg) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=block_icon)]
- [app/assets/images/active_admin/index_list_icons/blog_icon.svg](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/index_list_icons/blog_icon.svg) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=blog_icon)]
- [app/assets/images/active_admin/index_list_icons/grid_icon.svg](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/index_list_icons/grid_icon.svg) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=grid_icon)]
- [app/assets/images/active_admin/index_list_icons/table_icon.svg](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/index_list_icons/table_icon.svg) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=table_icon)]
- [app/assets/images/active_admin/loading.gif](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/loading.gif) [[search](https://github.com/activeadmin/activeadmin/search?utf8=%E2%9C%93&q=loading)]