Optimize query for issue neighbors

Remove optimized_issue_neighbor_queries feature flag so that the code
will use the new query which is already tested in production

Changelog: performance

app/models/issue.rb

@@ -233,8 +233,6 @@ class Issue < ApplicationRecord
   end
 
   def next_object_by_relative_position(ignoring: nil, order: :asc)
-    return super unless Feature.enabled?(:optimized_issue_neighbor_queries, project, default_enabled: :yaml)
-
     array_mapping_scope = -> (id_expression) do
       relation = Issue.where(Issue.arel_table[:project_id].eq(id_expression))
 

config/feature_flags/development/optimized_issue_neighbor_queries.yml

----
-name: optimized_issue_neighbor_queries
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76073
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/345921
-milestone: '14.6'
-type: development
-group: group::project management
-default_enabled: false

spec/models/issue_spec.rb

@@ -1328,28 +1328,10 @@ RSpec.describe Issue do
     let_it_be(:issue1) { create(:issue, project: project, relative_position: nil) }
     let_it_be(:issue2) { create(:issue, project: project, relative_position: nil) }
 
-    context 'when optimized_issue_neighbor_queries is enabled' do
-      before do
-        stub_feature_flags(optimized_issue_neighbor_queries: true)
-      end
-
-      it_behaves_like "a class that supports relative positioning" do
-        let_it_be(:project) { reusable_project }
-        let(:factory) { :issue }
-        let(:default_params) { { project: project } }
-      end
-    end
-
-    context 'when optimized_issue_neighbor_queries is disabled' do
-      before do
-        stub_feature_flags(optimized_issue_neighbor_queries: false)
-      end
-
-      it_behaves_like "a class that supports relative positioning" do
-        let_it_be(:project) { reusable_project }
-        let(:factory) { :issue }
-        let(:default_params) { { project: project } }
-      end
+    it_behaves_like "a class that supports relative positioning" do
+      let_it_be(:project) { reusable_project }
+      let(:factory) { :issue }
+      let(:default_params) { { project: project } }
     end
 
     it 'is not blocked for repositioning by default' do