Remove obsolete ActiveRecord::ExplainSubscriber reference for Rails 8.1

Rails 8.1 removed the ExplainSubscriber class (commit f488878f1bc),
causing test failures with: NameError: uninitialized constant
ActiveRecord::ExplainSubscriber

The ExplainSubscriber functionality was refactored into ExplainRegistry
with lazy subscription - instead of subscribing at initialization,
Rails 8.1 now subscribes only when .explain is first called via
ExplainRegistry.start.

The spec_helper.rb line that manually subscribed to ExplainSubscriber
is no longer needed or valid. Rails 8.1 handles the subscription
automatically and lazily.

Related Rails commit:
- f488878f1bc "Refactor ExplainRegistry to only be subscribed once used"
- Author: Jean Boussier
- Date: Thu Sep 25 10:37:09 2025 +0200
- Link:
rails/rails@f488878

Author: andynu

spec/spec_helper.rb

@@ -53,7 +53,6 @@ def set_logger
     ActiveSupport::Notifications.notifier = @notifier
 
     ActiveRecord::LogSubscriber.attach_to(:active_record)
-    ActiveSupport::Notifications.subscribe("sql.active_record", ActiveRecord::ExplainSubscriber.new)
   end
 
   class MockLogger