Remove feature flag for Group Projects API Plan Preloading

Removes feature flag `Group Projects API Plan Preloading`. This flag
has been enabled on GitLab.com since Jan 26 without incident.

Changelog: other
EE: true

config/feature_flags/development/group_project_api_preload_plans.yml

----
-name: group_project_api_preload_plans
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77538
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/350176
-milestone: '14.7'
-type: development
-group: group::authentication and authorization
-default_enabled: false

ee/app/models/ee/preloaders/single_hierarchy_project_group_plans_preloader.rb

@@ -4,7 +4,6 @@ module EE
   module Preloaders
     module SingleHierarchyProjectGroupPlansPreloader
       def execute
-        return unless ::Feature.enabled?(:group_project_api_preload_plans, default_enabled: :yaml)
         return unless ::Gitlab::CurrentSettings.should_check_namespace_plan?
         return unless project = projects.take
 

ee/spec/requests/api/groups_spec.rb

@@ -654,20 +654,6 @@ RSpec.describe API::Groups do
 
           expect(response).to have_gitlab_http_status(:ok)
         end
-
-        context 'when the feature flag is disabled' do
-          before do
-            stub_feature_flags(group_project_api_preload_plans: false)
-          end
-
-          it 'does not preload plans' do
-            expect(Plan).to receive(:hosted_plans_for_namespaces).at_least(:twice).and_call_original
-
-            get api("/groups/#{group.id}/projects", user), params: { include_subgroups: true }
-
-            expect(response).to have_gitlab_http_status(:ok)
-          end
-        end
       end
 
       context 'when there are no projects' do