Enable linear version User#membership_groups

In this commit we're removing the ff
`linear_user_membership_groups` and enabling the
linear version of `User#membership_groups`.

Changelog: changed

app/models/user.rb

@@ -991,11 +991,7 @@ class User < ApplicationRecord
 
   # Returns the groups a user is a member of, either directly or through a parent group
   def membership_groups
-    if Feature.enabled?(:linear_user_membership_groups, self, default_enabled: :yaml)
-      groups.self_and_descendants
-    else
-      Gitlab::ObjectHierarchy.new(groups).base_and_descendants
-    end
+    groups.self_and_descendants
   end
 
   # Returns a relation of groups the user has access to, including their parent

config/feature_flags/development/linear_user_membership_groups.yml

----
-name: linear_user_membership_groups
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68842
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339432
-milestone: '14.3'
-type: development
-group: group::access
-default_enabled: false

spec/models/user_spec.rb

@@ -3479,19 +3479,7 @@ RSpec.describe User do
 
     subject { user.membership_groups }
 
-    shared_examples 'returns groups where the user is a member' do
-      specify { is_expected.to contain_exactly(parent_group, child_group) }
-    end
-
-    it_behaves_like 'returns groups where the user is a member'
-
-    context 'when feature flag :linear_user_membership_groups is disabled' do
-      before do
-        stub_feature_flags(linear_user_membership_groups: false)
-      end
-
-      it_behaves_like 'returns groups where the user is a member'
-    end
+    specify { is_expected.to contain_exactly(parent_group, child_group) }
   end
 
   describe '#authorizations_for_projects' do