Remove IgnoreColumn from GeoNode

`system_hook_id` was removed in gitlab-org/gitlab-ee!2644.

This removed the `ignore_column` from GeoNode. And will be merged in
a later release (= 10.1).

app/models/geo_node.rb

 class GeoNode < ActiveRecord::Base
   include Presentable
-  include IgnorableColumn
-
-  ignore_column :system_hook_id
 
   belongs_to :geo_node_key, dependent: :destroy # rubocop: disable Cop/ActiveRecordDependent
   belongs_to :oauth_application, class_name: 'Doorkeeper::Application', dependent: :destroy # rubocop: disable Cop/ActiveRecordDependent