Enable linear version of `groups_including_descendants_by`

In this commit we're removing the ff
`linear_group_including_descendants_by` and enabling the
linear version of `Group.groups_including_descendants_by`.

Changelog: changed

app/models/group.rb

@@ -852,15 +852,7 @@ class Group < Namespace
   end
 
   def self.groups_including_descendants_by(group_ids)
-    groups = Group.where(id: group_ids)
-
-    if Feature.enabled?(:linear_group_including_descendants_by, default_enabled: :yaml)
-      groups.self_and_descendants
-    else
-      Gitlab::ObjectHierarchy
-      .new(groups)
-      .base_and_descendants
-    end
+    Group.where(id: group_ids).self_and_descendants
   end
 
   def disable_shared_runners!

config/feature_flags/development/linear_group_including_descendants_by.yml

----
-name: linear_group_including_descendants_by
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68835
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339431
-milestone: '14.3'
-type: development
-group: group::access
-default_enabled: false

spec/models/group_spec.rb

@@ -2317,14 +2317,6 @@ RSpec.describe Group do
     end
 
     it_behaves_like 'returns the expected groups for a group and its descendants'
-
-    context 'when :linear_group_including_descendants_by feature flag is disabled' do
-      before do
-        stub_feature_flags(linear_group_including_descendants_by: false)
-      end
-
-      it_behaves_like 'returns the expected groups for a group and its descendants'
-    end
   end
 
   describe '.preset_root_ancestor_for' do