Remove FF ci_auto_cancel_all_pipelines

It was enabled by default

config/feature_flags/development/ci_auto_cancel_all_pipelines.yml

----
-name: ci_auto_cancel_all_pipelines
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46686
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/275997
-milestone: '13.6'
-type: development
-group: group::pipeline authoring
-default_enabled: true

lib/gitlab/ci/pipeline/chain/cancel_pending_pipelines.rb

@@ -25,7 +25,7 @@ module Gitlab
 
           # rubocop: disable CodeReuse/ActiveRecord
           def auto_cancelable_pipelines
-            pipelines
+            project.all_pipelines.ci_and_parent_sources
               .where(ref: pipeline.ref)
               .where.not(id: pipeline.same_family_pipeline_ids)
               .where.not(sha: project.commit(pipeline.ref).try(:id))
@@ -33,14 +33,6 @@ module Gitlab
               .with_only_interruptible_builds
           end
           # rubocop: enable CodeReuse/ActiveRecord
-
-          def pipelines
-            if ::Feature.enabled?(:ci_auto_cancel_all_pipelines, project, default_enabled: true)
-              project.all_pipelines.ci_and_parent_sources
-            else
-              project.ci_pipelines
-            end
-          end
         end
       end
     end

spec/lib/gitlab/ci/pipeline/chain/cancel_pending_pipelines_spec.rb

@@ -58,20 +58,6 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::CancelPendingPipelines do
 
             expect(build_statuses(child_pipeline)).to contain_exactly('canceled')
           end
-
-          context 'when FF ci_auto_cancel_all_pipelines is disabled' do
-            before do
-              stub_feature_flags(ci_auto_cancel_all_pipelines: false)
-            end
-
-            it 'does not cancel interruptible builds of child pipeline' do
-              expect(build_statuses(child_pipeline)).to contain_exactly('running')
-
-              perform
-
-              expect(build_statuses(child_pipeline)).to contain_exactly('running')
-            end
-          end
         end
 
         context 'when the child pipeline has not an interruptible job' do