Use ldap_settings_unlock_groups_by_owners flag

ee/app/views/groups/ldap_group_settings/_form.html.haml

-- if ::Gitlab::CurrentSettings.allow_group_owners_to_manage_ldap
+- if Feature.enabled?(:ldap_settings_unlock_groups_by_owners) && ::Gitlab::CurrentSettings.allow_group_owners_to_manage_ldap
   %section.ldap-group-links
     = form_for(group, url: group_ldap_settings_url(group), method: :put) do |f|
       .form-holder
@@ -10,4 +10,3 @@
                 = _('Allow owners to manually add users outside of LDAP')
           .col-sm-2
             = f.submit _('Save'), class: 'btn btn-success qa-save'
-