Remove merge_train_enforcement feature flag

What does this MR do?

Removes the merge_train_enforcement feature flag now that the feature has been rolled out and is stable. With the flag gone, a project's Merge train enforcement setting always takes effect.

The setting itself is unchanged — allow_bypass (default), enforce_for_all_users, and enforce_with_owner_override all keep working exactly as before.

Changes

• Deleted the flag definition ee/config/feature_flags/wip/merge_train_enforcement.yml
• Removed the Feature.enabled?(:merge_train_enforcement, …) gates:
  • ProjectCiCdSetting#merge_train_enforced_for? (enforcement now driven purely by the setting)
  • GraphQL ProjectCiCdSettingsUpdate mutation (merge_train_enforcement is always accepted)
  • the merge-trains settings view (radio options always rendered)
• Dropped the "Ignored unless the merge_train_enforcement feature flag is enabled" note from the GraphQL field/argument descriptions
• Removed the now-obsolete "flag disabled" spec contexts
• Regenerated GraphQL docs, schema and introspection artifacts

Testing

Ran locally on the changed code — all green:

• ee/spec/models/project_ci_cd_setting_spec.rb
• ee/spec/graphql/ee/mutations/ci/project_ci_cd_settings_update_spec.rb
• spec/graphql/types/ci/ci_cd_setting_type_spec.rb
• ee/spec/serializers/ee/merge_request_poll_widget_entity_spec.rb

(The ECONNREFUSED localhost:80 failures in merge_service_spec reproduce identically on master without this change — a local-env artifact, unrelated.)

References #603364 (closed)