Remove unused ES application settings

Summary

Remove ES application settings that were moved to the ElasticsearchIndex model in !17230 (closed).

Note: This should be done in M+1 releases after the above MR is merged, following the process at https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns

Changes

• In release M+1, remove these columns from the application_settings table:

  • elasticsearch_url
  • elasticsearch_shards
  • elasticsearch_replicas
  • elasticsearch_aws
  • elasticsearch_aws_region
  • elasticsearch_aws_access_key
  • encrypted_elasticsearch_aws_secret_access_key
  • encrypted_elasticsearch_aws_secret_access_key_iv

• In release M+2, remove the ignore_columns rules in app/models/application_setting.rb.