Remove obsolete code about banner images and styles

We forgot to remove it in commit 8066b96fe. We're removing it now
because these methods didn't follow the Layout/BlockAlignment rubocop
rule, which we're about to add.

app/controllers/admin/banners_controller.rb

@@ -47,18 +47,6 @@ class Admin::BannersController < Admin::BaseController
        web_section_ids: []]
     end
 
-    def banner_styles
-      @banner_styles = Setting.all.banner_style.map do |banner_style|
-                         [banner_style.value, banner_style.key.split(".")[1]]
-                       end
-    end
-
-    def banner_imgs
-      @banner_imgs = Setting.all.banner_img.map do |banner_img|
-                       [banner_img.value, banner_img.key.split(".")[1]]
-                     end
-    end
-
     def banner_sections
       @banner_sections = WebSection.all
     end