Remove ci_dynamic_child_pipeline feature flag

Changelog: other
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63329

config/feature_flags/development/ci_dynamic_child_pipeline.yml

----
-name: ci_dynamic_child_pipeline
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/23790
-rollout_issue_url: 
-milestone: '12.9'
-type: development
-group: group::pipeline execution
-default_enabled: true

lib/gitlab/ci/config/external/file/artifact.rb

@@ -28,11 +28,6 @@ module Gitlab
               end
             end
 
-            def matching?
-              super &&
-                Feature.enabled?(:ci_dynamic_child_pipeline, project, default_enabled: true)
-            end
-
             private
 
             def project