Revert "Try to reset the migration properly"

This reverts commit 81d44ae4.

db/post_migrate/20170828170516_remove_user_external_mail_columns.rb

@@ -6,11 +6,7 @@ class RemoveUserExternalMailColumns < ActiveRecord::Migration
 
   DOWNTIME = false
 
-  def up
+  def change
     remove_column :users, :external_email, :boolean
   end
-
-  def down
-    add_column :users, :external_email, :boolean
-  end
 end