Remove merge_request_eager_fetch_ref feature flag

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80876 fixed a
problem where the Gitaly `FetchSourceBranch` RPC would cause the
database to idle in transaction. This happened because the call was
made in an `after_save` hook, which wraps everything in a transaction.
On GitLab.com, the 15-second idle in transaction timeout would close
the connection, but this was only apparent to the application when it
attempted to write a merge request metrics entry for a non-existent
merge request, resulting in a foreign key violation.

With the feature flag enabled, the `FetchSourceBranch` occurs outside
this hook. Since enabling this feature flag, we no longer see this
idle in transaction problem.

Relates to:

1. https://gitlab.com/gitlab-org/gitlab/-/issues/336657
2. https://gitlab.com/gitlab-org/gitlab/-/issues/353044

app/models/merge_request.rb

@@ -1026,12 +1026,13 @@ class MergeRequest < ApplicationRecord
     ensure_target_project_iid!
 
     fetch_ref!
-    # Prevent the after_create hook from fetching the source branch again
-    # Drop this field after rollout in https://gitlab.com/gitlab-org/gitlab/-/issues/353044.
+    # Prevent the after_create hook from fetching the source branch again.
     @skip_fetch_ref = true
   end
 
   def create_merge_request_diff
+    # Callers such as MergeRequests::BuildService may not call eager_fetch_ref!. Just
+    # in case they haven't, we fetch the ref.
     fetch_ref! unless skip_fetch_ref
 
     # n+1: https://gitlab.com/gitlab-org/gitlab/-/issues/19377

app/services/merge_requests/create_service.rb

@@ -36,9 +36,7 @@ module MergeRequests
       # callback (e.g. after_create), a database transaction will be
       # open while the Gitaly RPC waits. To avoid an idle in transaction
       # timeout, we do this before we attempt to save the merge request.
-      if Feature.enabled?(:merge_request_eager_fetch_ref, @project, default_enabled: :yaml)
-        merge_request.eager_fetch_ref!
-      end
+      merge_request.eager_fetch_ref!
     end
 
     def set_projects!

config/feature_flags/development/merge_request_eager_fetch_ref.yml

----
-name: merge_request_eager_fetch_ref
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80876
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/353044
-milestone: '14.9'
-type: development
-group: group::code review
-default_enabled: false

spec/services/merge_requests/create_service_spec.rb

@@ -454,7 +454,7 @@ RSpec.describe MergeRequests::CreateService, :clean_gitlab_redis_shared_state do
       end
     end
 
-    shared_examples 'when source and target projects are different' do |eager_fetch_ref_enabled|
+    shared_examples 'when source and target projects are different' do
       let(:target_project) { fork_project(project, nil, repository: true) }
 
       let(:opts) do
@@ -498,11 +498,7 @@ RSpec.describe MergeRequests::CreateService, :clean_gitlab_redis_shared_state do
 
         it 'creates the merge request', :sidekiq_might_not_need_inline do
           expect_next_instance_of(MergeRequest) do |instance|
-            if eager_fetch_ref_enabled
-              expect(instance).to receive(:eager_fetch_ref!).and_call_original
-            else
-              expect(instance).not_to receive(:eager_fetch_ref!)
-            end
+            expect(instance).to receive(:eager_fetch_ref!).and_call_original
           end
 
           merge_request = described_class.new(project: project, current_user: user, params: opts).execute
@@ -520,17 +516,7 @@ RSpec.describe MergeRequests::CreateService, :clean_gitlab_redis_shared_state do
       end
     end
 
-    context 'when merge_request_eager_fetch_ref is enabled' do
-      it_behaves_like 'when source and target projects are different', true
-    end
-
-    context 'when merge_request_eager_fetch_ref is disabled' do
-      before do
-        stub_feature_flags(merge_request_eager_fetch_ref: false)
-      end
-
-      it_behaves_like 'when source and target projects are different', false
-    end
+    it_behaves_like 'when source and target projects are different'
 
     context 'when user sets source project id' do
       let(:another_project) { create(:project) }