Views filter InOperator::getValueOptions() children should always return the stored value options

\Drupal\views\Plugin\views\filter\InOperator::getValueOptions() returns the stored value options and suggested children return the value options as well "if someone expects it". This resulted in confusing scenarios, because although this is a public method, its return value cannot be relied on. It may or may not have been the value options. Instead of this "suggestion", the return value expectation of InOperator children was changed to always return the stored value options.