Remove ci_always_refresh_merge_requests_from_beginning flag

This commit removes the feature flag.

changelogs/unreleased/remove-ci_always_refresh_merge_requests_from_beginning-flag.yml

+---
+title: Make the Merge Train process flow more resilient by always refreshing merge
+  requests from beginning
+merge_request: 46768
+author:
+type: fixed

config/feature_flags/development/ci_always_refresh_merge_requests_from_beginning.yml

----
-name: ci_always_refresh_merge_requests_from_beginning
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45232
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/268215
-milestone: '13.5'
-type: development
-group: group::continuous integration
-default_enabled: false

ee/app/services/merge_trains/refresh_merge_requests_service.rb

@@ -42,12 +42,13 @@ module MergeTrains
 
     attr_reader :merge_request
 
+    # NOTE:
+    # As we changed the process flow to refresh merge requests from the begnning always,
+    # we don't use the `items` argument anymore.
+    # We should refactor the current logic to make this class more readable.
+    # See https://gitlab.com/gitlab-org/gitlab/-/issues/281065
     def get_first_in_train(items)
-      if Feature.enabled?(:ci_always_refresh_merge_requests_from_beginning, merge_request.target_project)
-        MergeTrain.first_in_train(merge_request.target_project, merge_request.target_branch)
-      else
-        MergeTrain.first_in_train_from(items)
-      end
+      MergeTrain.first_in_train(merge_request.target_project, merge_request.target_branch)
     end
 
     def unsafe_refresh(first_merge_request)

ee/spec/services/merge_trains/refresh_merge_requests_service_spec.rb

@@ -181,62 +181,6 @@ RSpec.describe MergeTrains::RefreshMergeRequestsService do
         subject
       end
 
-      context 'when ci_always_refresh_merge_requests_from_beginning is disabled' do
-        before do
-          stub_feature_flags(ci_always_refresh_merge_requests_from_beginning: false)
-        end
-
-        it 'executes RefreshMergeRequestService to all the following merge requests' do
-          expect(refresh_service_1).not_to receive(:execute).with(merge_request_1)
-          expect(refresh_service_2).to receive(:execute).with(merge_request_2)
-
-          subject
-        end
-
-        context 'when merge request 1 was tried to be refreshed while the system is refreshing merge request 2' do
-          before do
-            allow_any_instance_of(described_class).to receive(:unsafe_refresh).with(merge_request_2) do
-              service.execute(merge_request_1)
-            end
-          end
-
-          it 'refreshes the merge request 1 later with AutoMergeProcessWorker' do
-            expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_1.id).once
-
-            subject
-          end
-
-          context 'when ci_always_refresh_merge_requests_from_beginning is disabled' do
-            before do
-              stub_feature_flags(ci_always_refresh_merge_requests_from_beginning: false)
-            end
-
-            it 'refreshes the merge request 1 later with AutoMergeProcessWorker' do
-              expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request_1.id).once
-
-              subject
-            end
-          end
-
-          it_behaves_like 'logging results', 4
-
-          context 'when merge request 1 has already been merged' do
-            before do
-              allow(merge_request_1.merge_train).to receive(:cleanup_ref)
-              merge_request_1.merge_train.update_column(:status, MergeTrain.state_machines[:status].states[:merged].value)
-            end
-
-            it 'does not refresh the merge request 1' do
-              expect(AutoMergeProcessWorker).not_to receive(:perform_async).with(merge_request_1.id)
-
-              subject
-            end
-
-            it_behaves_like 'logging results', 1
-          end
-        end
-      end
-
       it_behaves_like 'logging results', 3
     end
   end