Cleanup optimize_shifting_relative_positions flag

Removes the feature flag for optimizing calculation when shifting
siblings when moving nulls to start or end

Changelog: other
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60792

app/models/concerns/relative_positioning.rb

@@ -53,12 +53,7 @@ module RelativePositioning
 
       return [size, starting_from] if size >= MIN_GAP
 
-      terminus =
-        if Feature.enabled?(:optimize_shifting_relative_positions, default_enabled: :yaml)
-          context.at_position(starting_from)
-        else
-          at_end ? context.max_sibling : context.min_sibling
-        end
+      terminus = context.at_position(starting_from)
 
       if at_end
         terminus.shift_left

changelogs/unreleased/328402-remove-optimize-shifting-positions-feature-flag.yml

+---
+title: Remove optimize_shifting_relative_positions feature flag
+merge_request: 60792
+author:
+type: other

config/feature_flags/development/optimize_shifting_relative_positions.yml

----
-name: optimize_shifting_relative_positions
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59745
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/328402
-milestone: '13.12'
-type: development
-group: group::project management
-default_enabled: false

spec/support/shared_examples/models/relative_positioning_shared_examples.rb

@@ -66,7 +66,7 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
-  shared_examples '.move_nulls_to_end' do
+  describe '.move_nulls_to_end' do
     let(:item3) { create_item }
     let(:sibling_query) { item1.class.relative_positioning_query_base(item1) }
 
@@ -186,7 +186,7 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
-  shared_examples '.move_nulls_to_start' do
+  describe '.move_nulls_to_start' do
     let(:item3) { create_item }
     let(:sibling_query) { item1.class.relative_positioning_query_base(item1) }
 
@@ -261,24 +261,6 @@ RSpec.shared_examples 'a class that supports relative positioning' do
     end
   end
 
-  context 'when optimize_shifting_relative_positions is enabled' do
-    before do
-      stub_feature_flags(optimize_shifting_relative_positions: true)
-    end
-
-    it_behaves_like '.move_nulls_to_start'
-    it_behaves_like '.move_nulls_to_end'
-  end
-
-  context 'when optimize_shifting_relative_positions is disabled' do
-    before do
-      stub_feature_flags(optimize_shifting_relative_positions: false)
-    end
-
-    it_behaves_like '.move_nulls_to_start'
-    it_behaves_like '.move_nulls_to_end'
-  end
-
   describe '#move_before' do
     let(:item3) { create(factory, default_params) }