Remove the `validate_namespace_parent_type` feature flag

This change removes the
`validate_namespace_parent_type` feature flag
from the codebase

Changelog:  other

app/models/namespace.rb

@@ -96,7 +96,7 @@ class Namespace < ApplicationRecord
 
   validates :max_artifacts_size, numericality: { only_integer: true, greater_than: 0, allow_nil: true }
 
-  validate :validate_parent_type, if: -> { Feature.enabled?(:validate_namespace_parent_type, default_enabled: :yaml) }
+  validate :validate_parent_type
 
   # ProjectNamespaces excluded as they are not meant to appear in the group hierarchy at the moment.
   validate :nesting_level_allowed, unless: -> { project_namespace? }

config/feature_flags/development/validate_namespace_parent_type.yml

----
-name: validate_namespace_parent_type
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54094
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/322101
-milestone: '13.10'
-type: development
-group: group::access
-default_enabled: true

doc/user/group/index.md

@@ -805,13 +805,5 @@ the following checks when creating or updating namespaces or groups:
 - Namespaces must not have parents.
 - Group parents must be groups and not namespaces.
 
-You can disable the validation if GitLab shows the following errors:
-
-- `A user namespace cannot have a parent`.
-- `A group cannot have a user namespace as its parent`.
-
-To disable the validation,
-[disable the `validate_namespace_parent_type` flag](../../administration/feature_flags.md).
-
-In the unlikely event that you had to disable this feature flag to prevent errors,
+In the unlikely event that you see these errors in your GitLab installation,
 [contact Support](https://about.gitlab.com/support/) so that we can improve this validation.

spec/models/group_spec.rb

@@ -94,34 +94,6 @@ RSpec.describe Group do
           expect(group).to be_valid
         end
       end
-
-      context 'when the feature flag `validate_namespace_parent_type` is disabled' do
-        before do
-          stub_feature_flags(validate_namespace_parent_type: false)
-        end
-
-        context 'when the group has no parent' do
-          it 'allows a group to have no parent associated with it' do
-            group = build(:group)
-
-            expect(group).to be_valid
-          end
-        end
-
-        context 'when the group has a parent' do
-          it 'allows a group to have a namespace as its parent' do
-            group = build(:group, parent: build(:namespace))
-
-            expect(group).to be_valid
-          end
-
-          it 'allows a group to have another group as its parent' do
-            group = build(:group, parent: build(:group))
-
-            expect(group).to be_valid
-          end
-        end
-      end
     end
 
     describe 'path validation' do

spec/models/namespace_spec.rb

@@ -107,34 +107,6 @@ RSpec.describe Namespace do
           end
         end
       end
-
-      context 'when the feature flag `validate_namespace_parent_type` is disabled' do
-        before do
-          stub_feature_flags(validate_namespace_parent_type: false)
-        end
-
-        context 'when the namespace has no parent' do
-          it 'allows a namespace to have no parent associated with it' do
-            namespace = build(:namespace)
-
-            expect(namespace).to be_valid
-          end
-        end
-
-        context 'when the namespace has a parent' do
-          it 'allows a namespace to have a group as its parent' do
-            namespace = build(:namespace, parent: build(:group))
-
-            expect(namespace).to be_valid
-          end
-
-          it 'allows a namespace to have another namespace as its parent' do
-            namespace = build(:namespace, parent: build(:namespace))
-
-            expect(namespace).to be_valid
-          end
-        end
-      end
     end
 
     describe '#nesting_level_allowed' do