Remove system_hook_id from geo_nodes as per development guidelines

Instead of dropping the column in a migration, ignore the column and
drop it in a post-deploy migration.

A follow-up MR for 10.1 will be made to remove the `ignore_column`
from GeoNode.

See:
https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns

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

spec/ee/spec/migrations/remove_system_hook_from_geo_nodes_spec.rb

 require "spec_helper"
-require Rails.root.join("db", "migrate", "20170811082658_remove_system_hook_from_geo_nodes.rb")
+require Rails.root.join("db", "post_migrate", "20170811082658_remove_system_hook_from_geo_nodes.rb")
 
 describe RemoveSystemHookFromGeoNodes, :migration do
   let(:geo_nodes) { table(:geo_nodes) }