Remove FF ci_child_of_child_pipeline

This also removes the docs of it

app/models/ci/pipeline.rb

@@ -829,16 +829,9 @@ module Ci
     end
 
     def same_family_pipeline_ids
-      if ::Gitlab::Ci::Features.child_of_child_pipeline_enabled?(project)
-        ::Gitlab::Ci::PipelineObjectHierarchy.new(
-          base_and_ancestors(same_project: true), options: { same_project: true }
-        ).base_and_descendants.select(:id)
-      else
-        # If pipeline is a child of another pipeline, include the parent
-        # and the siblings, otherwise return only itself and children.
-        parent = parent_pipeline || self
-        [parent.id] + parent.child_pipelines.pluck(:id)
-      end
+      ::Gitlab::Ci::PipelineObjectHierarchy.new(
+        base_and_ancestors(same_project: true), options: { same_project: true }
+      ).base_and_descendants.select(:id)
     end
 
     def build_with_artifacts_in_self_and_descendants(name)

app/services/ci/create_downstream_pipeline_service.rb

@@ -77,16 +77,9 @@ module Ci
 
       # TODO: Remove this condition if favour of model validation
       # https://gitlab.com/gitlab-org/gitlab/issues/38338
-      if ::Gitlab::Ci::Features.child_of_child_pipeline_enabled?(project)
-        if has_max_descendants_depth?
-          @bridge.drop!(:reached_max_descendant_pipelines_depth)
-          return false
-        end
-      else
-        if @bridge.triggers_child_pipeline? && @bridge.pipeline.parent_pipeline.present?
-          @bridge.drop!(:bridge_pipeline_is_child_pipeline)
-          return false
-        end
+      if has_max_descendants_depth?
+        @bridge.drop!(:reached_max_descendant_pipelines_depth)
+        return false
       end
 
       unless can_create_downstream_pipeline?(target_ref)

config/feature_flags/development/ci_child_of_child_pipeline.yml

----
-name: ci_child_of_child_pipeline
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41102
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/243747
-group: group::continuous integration
-type: development
-default_enabled: true

doc/ci/parent_child_pipelines.md

@@ -164,36 +164,13 @@ This is [resolved in GitLab 12.10](https://gitlab.com/gitlab-org/gitlab/-/issues
 ## Nested child pipelines
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/29651) in GitLab 13.4.
-> - It's [deployed behind a feature flag](../user/feature_flags.md), enabled by default.
-> - It's enabled on GitLab.com.
-> - It's recommended for production use.
-> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-nested-child-pipelines). **(CORE ONLY)**
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/243747) in GitLab 13.5.
 
 Parent and child pipelines were introduced with a maximum depth of one level of child
 pipelines, which was later increased to two. A parent pipeline can trigger many child
 pipelines, and these child pipelines can trigger their own child pipelines. It's not
 possible to trigger another level of child pipelines.
 
-### Enable or disable nested child pipelines **(CORE ONLY)**
-
-Nested child pipelines with a depth of two are under development but ready for
-production use. This feature is deployed behind a feature flag that is **enabled by default**.
-
-[GitLab administrators with access to the GitLab Rails console](../administration/feature_flags.md)
-can opt to disable it.
-
-To enable it:
-
-```ruby
-Feature.enable(:ci_child_of_child_pipeline)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:ci_child_of_child_pipeline)
-```
-
 ## Pass variables to a child pipeline
 
 You can [pass variables to a downstream pipeline](multi_project_pipelines.md#passing-variables-to-a-downstream-pipeline).

lib/gitlab/ci/features.rb

@@ -46,10 +46,6 @@ module Gitlab
         Feature.enabled?(:project_transactionless_destroy, project, default_enabled: false)
       end
 
-      def self.child_of_child_pipeline_enabled?(project)
-        ::Feature.enabled?(:ci_child_of_child_pipeline, project, default_enabled: true)
-      end
-
       def self.trace_overwrite?
         ::Feature.enabled?(:ci_trace_overwrite, type: :ops, default_enabled: false)
       end

spec/services/ci/create_downstream_pipeline_service_spec.rb

@@ -325,20 +325,6 @@ RSpec.describe Ci::CreateDownstreamPipelineService, '#execute' do
 
             expect(bridge.reload).to be_success
           end
-
-          context 'when FF ci_child_of_child_pipeline is disabled' do
-            before do
-              stub_feature_flags(ci_child_of_child_pipeline: false)
-            end
-
-            it 'does not create a further child pipeline' do
-              expect { service.execute(bridge) }
-                .not_to change { Ci::Pipeline.count }
-
-              expect(bridge.reload).to be_failed
-              expect(bridge.failure_reason).to eq 'bridge_pipeline_is_child_pipeline'
-            end
-          end
         end
 
         context 'when upstream pipeline has a parent pipeline, which has a parent pipeline' do