Remove Elasticsearch settings from application settings

Description

Context

We moved some Elasticsearch application settings to a new JSONB column !174172 (merged).

In that migration, we also dropped the original columns without following the multi-milestone release process. This caused an incident: gitlab-com/gl-infra/production#19079 (closed).

We updated the last migration, keeping the original columns !176987 (merged).

At the moment, we have the original columns and the JSONB columns with a jsob accessor. When we update any setting, it gets updated in both places. Now, we will follow the process to drop columns: https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns

Current state

This removes the Elasticsearch settings columns from application_settings table. This is the second step after ignoring the columns !177568 (merged).

Related to #511921 (closed).