Remove deprecated attributes from AdminNotification

Some fields from AdminNotification are translatable and we no longer
need them. This commit will remove the annoying deprecation warning
thrown by Globalize gem after gem version update.

db/migrate/20190221163818_remove_deprecated_translatable_fields_from_admin_notifications.rb

@@ -0,0 +1,6 @@
+class RemoveDeprecatedTranslatableFieldsFromAdminNotifications < ActiveRecord::Migration[4.2]
+  def change
+    remove_column :admin_notifications, :title, :string
+    remove_column :admin_notifications, :body, :text
+  end
+end

db/schema.rb

@@ -55,8 +55,6 @@ ActiveRecord::Schema.define(version: 20190607160900) do
   end
 
   create_table "admin_notifications", force: :cascade do |t|
-    t.string   "title"
-    t.text     "body"
     t.string   "link"
     t.string   "segment_recipient"
     t.integer  "recipients_count"