Remove pages_serve_from_deployments FF

It was enabled for a couple of releases already

app/models/pages/lookup_path.rb

@@ -43,8 +43,6 @@ module Pages
 
     def deployment
       strong_memoize(:deployment) do
-        next unless Feature.enabled?(:pages_serve_from_deployments, project, default_enabled: true)
-
         project.pages_metadatum.pages_deployment
       end
     end

config/feature_flags/development/pages_serve_from_deployments.yml

----
-name: pages_serve_from_deployments
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46320
-rollout_issue_url: https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2932
-milestone: '13.6'
-type: development
-group: group::release
-default_enabled: true

spec/models/pages/lookup_path_spec.rb

@@ -117,14 +117,6 @@ RSpec.describe Pages::LookupPath do
         end
       end
 
-      context 'when pages_serve_from_deployments feature flag is disabled' do
-        before do
-          stub_feature_flags(pages_serve_from_deployments: false)
-        end
-
-        include_examples 'uses disk storage'
-      end
-
       context 'when deployment were created during migration' do
         before do
           allow(deployment).to receive(:migrated?).and_return(true)