Remove leftover database columns from Devise::Models::Rememberable (#17191)

* Remove leftover database columns from Devise::Models::Rememberable

* Update fix-duplication maintenance script

* Improve errors/warnings in the fix-duplicates maintenance script

db/post_migrate/20220118183123_remove_rememberable_from_users.rb

@@ -0,0 +1,8 @@
+class RemoveRememberableFromUsers < ActiveRecord::Migration[6.1]
+  def change
+    safety_assured do
+      remove_column :users, :remember_token, :string, null: true, default: nil
+      remove_column :users, :remember_created_at, :datetime, null: true, default: nil
+    end
+  end
+end