Logging: Delete backend code for configuring logging via Elastic Stack

Why are we doing this work

In %15.0, we disabled/removed pod logs by moving it behind a feature flag.

⚠ Removal should take place in %15.2 or later.

Tracking issue for all logging removal work: Monitor:Logging Deprecation (#346485 - closed)

Relevant links

• app/controllers/clusters/clusters_controller.rb
• app/helpers/environments_helper.rb
• app/models/clusters/cluster.rb
• app/models/clusters/applications/elastic_stack.rb
• app/models/clusters/applications/elastic_stack.rb
• app/models/clusters/integrations/elastic_stack.rb
• app/models/clusters/concerns/elasticsearch_client.rb
• app/presenters/clusters/integration_presenter.rb
• app/models/environment.rb
• app/presenters/clusters/cluster_presenter.rb
• app/services/clusters/integrations/create_service.rb
• .rubocop_todo/layout/line_length.yml
• app/serializers/cluster_entity.rb

Implementation plan

1. Delete Elastic Stack routes/controllers/models, application code
2. Include ~"group::configure" in code review

Usage metrics and database cleanup are out of scope. See Logging: Deprecate usage metrics for ElasticSta... (#360195 - closed), Logging: Cleanup database of `clusters_applicat... (#360196 - closed).

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