Remove FF ci_parent_child_pipeline

This commit removes the FF

lib/gitlab/ci/config/entry/trigger.rb

@@ -25,8 +25,7 @@ module Gitlab
             strategy :CrossProjectTrigger, if: -> (config) { !config.key?(:include) }
 
             strategy :SameProjectTrigger, if: -> (config) do
-              ::Feature.enabled?(:ci_parent_child_pipeline, default_enabled: true) &&
-                config.key?(:include)
+              config.key?(:include)
             end
 
             class CrossProjectTrigger < ::Gitlab::Config::Entry::Node
@@ -72,11 +71,7 @@ module Gitlab
 
             class UnknownStrategy < ::Gitlab::Config::Entry::Node
               def errors
-                if ::Feature.enabled?(:ci_parent_child_pipeline, default_enabled: true)
-                  ['config must specify either project or include']
-                else
-                  ['config must specify project']
-                end
+                ['config must specify either project or include']
               end
             end
           end

spec/lib/gitlab/ci/config/entry/trigger_spec.rb

@@ -114,19 +114,6 @@ describe Gitlab::Ci::Config::Entry::Trigger do
             .to match /config contains unknown keys: branch/
         end
       end
-
-      context 'when feature flag is off' do
-        before do
-          stub_feature_flags(ci_parent_child_pipeline: false)
-        end
-
-        let(:config) { { include: 'path/to/config.yml' } }
-
-        it 'is returns an error if include is used' do
-          expect(subject.errors.first)
-            .to match /config must specify project/
-        end
-      end
     end
 
     context 'when config contains unknown keys' do