Remove feature flag used to enable subtransactions counter

Changelog: performance

config/feature_flags/ops/active_record_subtransactions_counter.yml

----
-name: active_record_subtransactions_counter
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66477
-rollout_issue_url:
-milestone: '14.1'
-type: ops
-group: group::pipeline execution
-default_enabled: false

lib/gitlab/database.rb

@@ -207,10 +207,10 @@ module Gitlab
       extend ActiveSupport::Concern
 
       class_methods do
-        # A monkeypatch over ActiveRecord::Base.transaction.
-        # It provides observability into transactional methods.
+        # A patch over ActiveRecord::Base.transaction that provides
+        # observability into transactional methods.
         def transaction(**options, &block)
-          if options[:requires_new] && gitlab_track_subtransactions?
+          if options[:requires_new] && connection.transaction_open?
             ::Gitlab::Database::Metrics.subtransactions_increment(self.name)
           end
 
@@ -218,11 +218,6 @@ module Gitlab
             super(**options, &block)
           end
         end
-
-        def gitlab_track_subtransactions?
-          ::Feature.enabled?(:active_record_subtransactions_counter, type: :ops, default_enabled: :yaml) &&
-            connection.transaction_open?
-        end
       end
     end