Remove ignore rule for seat_link_enabled in ApplicationSetting

In the previous milestone, the seat_link_enabled database column in
ApplicationSetting was removed. With the new milestone, the ignore rule
can now be safely removed as well.

app/models/application_setting.rb

@@ -9,7 +9,6 @@ class ApplicationSetting < ApplicationRecord
   include Sanitizable
 
   ignore_columns %i[elasticsearch_shards elasticsearch_replicas], remove_with: '14.4', remove_after: '2021-09-22'
-  ignore_column :seat_link_enabled, remove_with: '14.4', remove_after: '2021-09-22'
 
   INSTANCE_REVIEW_MIN_USERS = 50
   GRAFANA_URL_ERROR_MESSAGE = 'Please check your Grafana URL setting in ' \