Remove permitted_attributes_for_import_export feature flag

Changelog: changed

config/feature_flags/development/permitted_attributes_for_import_export.yml

----
-name: permitted_attributes_for_import_export
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70168
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/340789
-milestone: '14.4'
-type: development
-group: group::import
-default_enabled: true

lib/gitlab/import_export/base/relation_factory.rb

@@ -196,7 +196,7 @@ module Gitlab
         end
 
         def use_attributes_permitter?
-          Feature.enabled?(:permitted_attributes_for_import_export, default_enabled: :yaml)
+          true
         end
 
         def existing_or_new_object

lib/gitlab/import_export/group/relation_tree_restorer.rb

@@ -118,7 +118,7 @@ module Gitlab
         end
 
         def filter_attributes(params)
-          if use_attributes_permitter? && attributes_permitter.permitted_attributes_defined?(importable_class_sym)
+          if attributes_permitter.permitted_attributes_defined?(importable_class_sym)
             attributes_permitter.permit(importable_class_sym, params)
           else
             Gitlab::ImportExport::AttributeCleaner.clean(
@@ -132,10 +132,6 @@ module Gitlab
           @attributes_permitter ||= Gitlab::ImportExport::AttributesPermitter.new
         end
 
-        def use_attributes_permitter?
-          Feature.enabled?(:permitted_attributes_for_import_export, default_enabled: :yaml)
-        end
-
         def present_override_params
           # we filter out the empty strings from the overrides
           # keeping the default values configured