Remove deprecated attributes from Banner

Some attributes from Banner are translatable and we no longer need to
have them at database table. This will remove the annoying deprecation
warning thrown by Globalize gem after gem version update.

db/migrate/20190221154819_remove_deprecated_translatable_fields_from_banners.rb

@@ -0,0 +1,6 @@
+class RemoveDeprecatedTranslatableFieldsFromBanners < ActiveRecord::Migration[4.2]
+  def change
+    remove_column :banners, :title, :string
+    remove_column :banners, :description, :string
+  end
+end

db/schema.rb

@@ -105,14 +105,12 @@ ActiveRecord::Schema.define(version: 20190607160900) do
   end
 
   create_table "banners", force: :cascade do |t|
-    t.string   "title",            limit: 80
-    t.string   "description"
     t.string   "target_url"
     t.date     "post_started_at"
     t.date     "post_ended_at"
     t.datetime "hidden_at"
-    t.datetime "created_at",                  null: false
-    t.datetime "updated_at",                  null: false
+    t.datetime "created_at",       null: false
+    t.datetime "updated_at",       null: false
     t.text     "background_color"
     t.text     "font_color"
     t.index ["hidden_at"], name: "index_banners_on_hidden_at", using: :btree