Restore the ability to override the language.types configuration

For some reason, the language.types configuration was not allowed to be overridden.

Let's make it overridable again as there doesn't seem to be any valid reason to forbid it.

EDIT: In fact there was a good reason, as described in this issue: #3547172: Removed items are kept in overridden configuration.

Configuration override cannot guarantee the order of overridden items as required by the LanguageNegotiator class (check related core issue).

EDIT: You can still make language.types overridable using the following hook:

function my_module_domain_config_ui_disallowed_configurations_alter(&$disallowed_configurations) {
  if (($key = array_search('language.types', $disallowed_configurations, TRUE)) !== FALSE) {
    unset($disallowed_configurations[$key]);
  }
}