Disable the `type` STI column for the migration

There is no need to make a difference between a Group or User
namespace, so also no need to define the classes of those.

lib/gitlab/background_migration/backfill_project_fullpath_in_repo_config.rb

@@ -77,6 +77,7 @@ module Gitlab
       # child of another namespace.
       class Namespace < ActiveRecord::Base
         self.table_name = 'namespaces'
+        self.inheritance_column = nil
 
         include Routable