Remove the ci_pipeline_ensure_iid_on_drop feature flag

MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/59626
Issue: gitlab.com/gitlab-org/gitlab/-/issues/326886

changelogs/unreleased/326886-feature-flag-rollout-of-ci_pipeline_ensure_iid_on_drop.yml

+---
+title: Ensure the project iid is set before dropping pipeline
+merge_request: 59626
+author:
+type: performance

config/feature_flags/development/ci_pipeline_ensure_iid_on_drop.yml

----
-name: ci_pipeline_ensure_iid_on_drop
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57783
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326886
-milestone: '13.11'
-type: development
-group: group::code review
-default_enabled: false

lib/gitlab/ci/pipeline/chain/helpers.rb

@@ -31,12 +31,10 @@ module Gitlab
             return if pipeline.readonly?
 
             if drop_reason && command.save_incompleted
-              if Feature.enabled?(:ci_pipeline_ensure_iid_on_drop, pipeline.project, default_enabled: :yaml)
               # Project iid must be called outside a transaction, so we ensure it is set here
               # otherwise it may be set within the state transition transaction of the drop! call
               # which it will lock the InternalId row for the whole transaction
               pipeline.ensure_project_iid!
-              end
 
               pipeline.drop!(drop_reason)
             else

spec/lib/gitlab/ci/pipeline/chain/helpers_spec.rb

@@ -57,14 +57,5 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Helpers do
         end
       end
     end
-
-    context 'when the ci_pipeline_ensure_iid_on_drop feature flag is false' do
-      it 'does not ensure the project iid' do
-        stub_feature_flags(ci_pipeline_ensure_iid_on_drop: false)
-        expect(pipeline).to receive(:ensure_project_iid!).once
-
-        subject.error(message, config_error: true)
-      end
-    end
   end
 end