Enable new Rails connection handling

This is prep for multi-database connections in the future.

This is achieved by removing the production env override - so the
legacy_connection_handlina setting in config/application.rb takes
effect.

Changelog: other

config/environments/production.rb

@@ -54,9 +54,6 @@ Rails.application.configure do
   # Enable serving of images, stylesheets, and JavaScripts from an asset server
   config.action_controller.asset_host = ENV['GITLAB_CDN_HOST'] if ENV['GITLAB_CDN_HOST'].present?
 
-  # We use a env var to keep at old default until we enable this for GitLab.com
-  config.active_record.legacy_connection_handling = !Gitlab::Utils.to_boolean(ENV.fetch('ENABLE_RAILS_61_CONNECTION_HANDLING', false))
-
   # Do not dump schema after migrations.
   config.active_record.dump_schema_after_migration = false